<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:fc209fd4-fae3-43ec-89ab-9caf89128249,g:302cdba2-649e-44c8-b7a3-6179de519005-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/ConsolidatedBalanceSheets" xlink:type="simple" xlink:href="rnac-20251231.xsd#ConsolidatedBalanceSheets"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/ConsolidatedBalanceSheets" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_7576957e-8948-47ef-82a2-dba340a72b2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_82e697ab-7985-4aa0-978a-f92a1c50aa20" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_7576957e-8948-47ef-82a2-dba340a72b2e" xlink:to="loc_us-gaap_StatementTable_82e697ab-7985-4aa0-978a-f92a1c50aa20" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_6bb5820f-9377-472c-afd0-33084dabf9fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_82e697ab-7985-4aa0-978a-f92a1c50aa20" xlink:to="loc_us-gaap_StatementClassOfStockAxis_6bb5820f-9377-472c-afd0-33084dabf9fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_6bb5820f-9377-472c-afd0-33084dabf9fd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_6bb5820f-9377-472c-afd0-33084dabf9fd" xlink:to="loc_us-gaap_ClassOfStockDomain_6bb5820f-9377-472c-afd0-33084dabf9fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_03d7fd2e-4883-493f-8fe4-8adc725cf50b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_6bb5820f-9377-472c-afd0-33084dabf9fd" xlink:to="loc_us-gaap_ClassOfStockDomain_03d7fd2e-4883-493f-8fe4-8adc725cf50b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_e53407da-766e-4fa3-af47-4cacb2d00cec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_03d7fd2e-4883-493f-8fe4-8adc725cf50b" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_e53407da-766e-4fa3-af47-4cacb2d00cec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_7d6d621f-68ad-4e94-91be-db2c278780bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_03d7fd2e-4883-493f-8fe4-8adc725cf50b" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_7d6d621f-68ad-4e94-91be-db2c278780bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7576957e-8948-47ef-82a2-dba340a72b2e" xlink:to="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_489f491e-e08e-41e0-817e-f3d82bc7a35d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_AssetsCurrentAbstract_489f491e-e08e-41e0-817e-f3d82bc7a35d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_3aa08ede-dcf1-4ca2-b305-de11e95e0c11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_489f491e-e08e-41e0-817e-f3d82bc7a35d" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_3aa08ede-dcf1-4ca2-b305-de11e95e0c11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_743d6f31-1352-42a2-bb06-014fdddc5e89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_489f491e-e08e-41e0-817e-f3d82bc7a35d" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_743d6f31-1352-42a2-bb06-014fdddc5e89" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_cd3fb3b2-c1f5-42f4-8035-f2abd23b2d6c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_489f491e-e08e-41e0-817e-f3d82bc7a35d" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_cd3fb3b2-c1f5-42f4-8035-f2abd23b2d6c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_68ca41a1-c668-408e-89b0-2526d6502c23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_489f491e-e08e-41e0-817e-f3d82bc7a35d" xlink:to="loc_us-gaap_AssetsCurrent_68ca41a1-c668-408e-89b0-2526d6502c23" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_79318862-0c7d-44f7-b843-a9884ba5f9f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_79318862-0c7d-44f7-b843-a9884ba5f9f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_9d9053ae-1475-4dec-8a95-e6c4e218c04f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_9d9053ae-1475-4dec-8a95-e6c4e218c04f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_5407c710-daeb-4613-8278-e818362f5ef5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_5407c710-daeb-4613-8278-e818362f5ef5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_f07d63dc-411b-4fd9-9670-083587f6ef1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_Goodwill_f07d63dc-411b-4fd9-9670-083587f6ef1a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsNoncurrent_2d3a8056-0274-4886-81a5-c7802a3f34be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsNoncurrent_2d3a8056-0274-4886-81a5-c7802a3f34be" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_c500c4a5-3b38-4084-9dfd-5adca1cefc44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_c500c4a5-3b38-4084-9dfd-5adca1cefc44" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsNoncurrent_63d0746e-6f24-4df7-9baf-2a32bf8b19c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsNoncurrent_63d0746e-6f24-4df7-9baf-2a32bf8b19c0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_5b7f7378-5137-497c-b3de-f8c2d26e3bf8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_535f5921-774f-4dfc-9914-dee154534141" xlink:to="loc_us-gaap_Assets_5b7f7378-5137-497c-b3de-f8c2d26e3bf8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7576957e-8948-47ef-82a2-dba340a72b2e" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_0798c5d7-427b-471c-b71a-f3dc7105b4e3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_0798c5d7-427b-471c-b71a-f3dc7105b4e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_bc546a27-8138-48c8-81e3-ac786112a422" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_0798c5d7-427b-471c-b71a-f3dc7105b4e3" xlink:to="loc_us-gaap_AccountsPayableCurrent_bc546a27-8138-48c8-81e3-ac786112a422" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesAndOtherLiabilities_0eba8d94-b9fb-4a89-97fa-720f5881a5ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedLiabilitiesAndOtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_0798c5d7-427b-471c-b71a-f3dc7105b4e3" xlink:to="loc_us-gaap_AccruedLiabilitiesAndOtherLiabilities_0eba8d94-b9fb-4a89-97fa-720f5881a5ca" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_c24b3dac-05c2-4c0a-824e-af9d889b0244" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_0798c5d7-427b-471c-b71a-f3dc7105b4e3" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_c24b3dac-05c2-4c0a-824e-af9d889b0244" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightLiabilityCurrent_8550dc3d-6273-4366-9460-aabb3b155da3" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_0798c5d7-427b-471c-b71a-f3dc7105b4e3" xlink:to="loc_rnac_ContingentValueRightLiabilityCurrent_8550dc3d-6273-4366-9460-aabb3b155da3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_a4a8ee90-0155-47fe-8474-c987712654fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_0798c5d7-427b-471c-b71a-f3dc7105b4e3" xlink:to="loc_us-gaap_LiabilitiesCurrent_a4a8ee90-0155-47fe-8474-c987712654fb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_f48285a5-1ffa-4cc3-a8c8-c49a905d7d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_f48285a5-1ffa-4cc3-a8c8-c49a905d7d4e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_WarrantsAndRightsOutstandingNoncurrent_1f8cbbff-e7ff-450f-9d55-4a2655d34d9f" xlink:href="rnac-20251231.xsd#rnac_WarrantsAndRightsOutstandingNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_rnac_WarrantsAndRightsOutstandingNoncurrent_1f8cbbff-e7ff-450f-9d55-4a2655d34d9f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightLiabilityNoncurrent_9eb25604-a69b-4323-b95f-af240de36613" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_rnac_ContingentValueRightLiabilityNoncurrent_9eb25604-a69b-4323-b95f-af240de36613" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_3bf2422c-a12d-4377-b2c0-60a3c9b4eff8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_3bf2422c-a12d-4377-b2c0-60a3c9b4eff8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_e538cab2-8838-4eda-87d6-efff536b3a17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_us-gaap_Liabilities_e538cab2-8838-4eda-87d6-efff536b3a17" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_d39283e4-33a4-4bc2-9068-a877e56a9df4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_us-gaap_CommitmentsAndContingencies_d39283e4-33a4-4bc2-9068-a877e56a9df4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_fd26c924-8416-4294-8860-55db0a6f1e44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_us-gaap_StockholdersEquityAbstract_fd26c924-8416-4294-8860-55db0a6f1e44" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_23d2cf0c-025c-4980-aff5-ea49d3727054" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_fd26c924-8416-4294-8860-55db0a6f1e44" xlink:to="loc_us-gaap_PreferredStockValue_23d2cf0c-025c-4980-aff5-ea49d3727054" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_e711bedb-e3db-412a-bba0-0249944b9074" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_fd26c924-8416-4294-8860-55db0a6f1e44" xlink:to="loc_us-gaap_CommonStockValue_e711bedb-e3db-412a-bba0-0249944b9074" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_c43a3d11-5a56-4ecb-8913-6650b5e92778" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_fd26c924-8416-4294-8860-55db0a6f1e44" xlink:to="loc_us-gaap_AdditionalPaidInCapital_c43a3d11-5a56-4ecb-8913-6650b5e92778" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_c7ccb9b2-3288-4bd5-bfe3-87382129343c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_fd26c924-8416-4294-8860-55db0a6f1e44" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_c7ccb9b2-3288-4bd5-bfe3-87382129343c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_4154344b-279d-452d-8061-0c392395461f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_fd26c924-8416-4294-8860-55db0a6f1e44" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_4154344b-279d-452d-8061-0c392395461f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_136da144-50ef-47fb-a3a6-d6538545f39b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_fd26c924-8416-4294-8860-55db0a6f1e44" xlink:to="loc_us-gaap_StockholdersEquity_136da144-50ef-47fb-a3a6-d6538545f39b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_3b28365b-7bbc-413a-a1fd-2cee248cc109" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_aa17a8c9-5519-4f9e-81c9-60fb19f8499f" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_3b28365b-7bbc-413a-a1fd-2cee248cc109" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="simple" xlink:href="rnac-20251231.xsd#ConsolidatedBalanceSheetsParenthetical"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_642866b1-54d4-493d-8917-64d340473f81" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_StatementTable_642866b1-54d4-493d-8917-64d340473f81" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_a3d5c288-f750-4720-b201-b5b8441449d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_642866b1-54d4-493d-8917-64d340473f81" xlink:to="loc_us-gaap_StatementClassOfStockAxis_a3d5c288-f750-4720-b201-b5b8441449d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_a3d5c288-f750-4720-b201-b5b8441449d0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_a3d5c288-f750-4720-b201-b5b8441449d0" xlink:to="loc_us-gaap_ClassOfStockDomain_a3d5c288-f750-4720-b201-b5b8441449d0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_a0322486-49b2-403d-9a77-0258121719b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_a3d5c288-f750-4720-b201-b5b8441449d0" xlink:to="loc_us-gaap_ClassOfStockDomain_a0322486-49b2-403d-9a77-0258121719b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_8fe61cb5-fe3b-41ab-a4e9-504f2fcc681e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_a0322486-49b2-403d-9a77-0258121719b0" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_8fe61cb5-fe3b-41ab-a4e9-504f2fcc681e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_05962db0-3128-41a3-bbf1-a5a3c824210d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_a0322486-49b2-403d-9a77-0258121719b0" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_05962db0-3128-41a3-bbf1-a5a3c824210d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_1a4d7a59-4ef2-41ce-9856-395c267e1538" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_1a4d7a59-4ef2-41ce-9856-395c267e1538" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_75f9b3e7-5a3f-402f-9058-c8ecdc446ebc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_75f9b3e7-5a3f-402f-9058-c8ecdc446ebc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_91bc4a2d-57d3-4b71-b491-e3ddd77fc971" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_PreferredStockSharesIssued_91bc4a2d-57d3-4b71-b491-e3ddd77fc971" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_c6fc4054-3023-41d5-ac16-5d05bd975819" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_c6fc4054-3023-41d5-ac16-5d05bd975819" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_7a876633-d032-43e8-93ea-9c2835187970" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_7a876633-d032-43e8-93ea-9c2835187970" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_5bc27203-ab7a-4bc1-b24e-f08907d1f37d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_5bc27203-ab7a-4bc1-b24e-f08907d1f37d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_587a0b3b-5cb3-4b5e-9e25-1ac0d7655430" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_CommonStockSharesIssued_587a0b3b-5cb3-4b5e-9e25-1ac0d7655430" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_77900052-a9a1-4864-8652-22fd29aef345" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136eb863-48cb-4fbf-8238-8de627679aba" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_77900052-a9a1-4864-8652-22fd29aef345" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/ConsolidatedStatementsofChangesinConvertiblePreferredStockandStockholdersEquityDeficit" xlink:type="simple" xlink:href="rnac-20251231.xsd#ConsolidatedStatementsofChangesinConvertiblePreferredStockandStockholdersEquityDeficit"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/ConsolidatedStatementsofChangesinConvertiblePreferredStockandStockholdersEquityDeficit" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_eba0c72a-f6c6-46e8-98f0-2f79c6b85954" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_ebfd6bbe-35ac-4a29-b7d4-fbb3ce703fb3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_eba0c72a-f6c6-46e8-98f0-2f79c6b85954" xlink:to="loc_us-gaap_StatementTable_ebfd6bbe-35ac-4a29-b7d4-fbb3ce703fb3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_d2121dd1-43a9-4e7a-94ac-251882ec58ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_ebfd6bbe-35ac-4a29-b7d4-fbb3ce703fb3" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_d2121dd1-43a9-4e7a-94ac-251882ec58ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_d2121dd1-43a9-4e7a-94ac-251882ec58ad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_d2121dd1-43a9-4e7a-94ac-251882ec58ad" xlink:to="loc_us-gaap_EquityComponentDomain_d2121dd1-43a9-4e7a-94ac-251882ec58ad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_e4c81c75-e444-4909-824a-ad9b2b9dfedb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_d2121dd1-43a9-4e7a-94ac-251882ec58ad" xlink:to="loc_us-gaap_EquityComponentDomain_e4c81c75-e444-4909-824a-ad9b2b9dfedb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_b777b118-912b-4bf1-a1ef-9932c8c3a58b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e4c81c75-e444-4909-824a-ad9b2b9dfedb" xlink:to="loc_us-gaap_PreferredStockMember_b777b118-912b-4bf1-a1ef-9932c8c3a58b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_86992535-9624-4584-b01c-603ff5b6a34a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e4c81c75-e444-4909-824a-ad9b2b9dfedb" xlink:to="loc_us-gaap_CommonStockMember_86992535-9624-4584-b01c-603ff5b6a34a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_5d1a0b75-32f7-4224-b4a4-9de4256787de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e4c81c75-e444-4909-824a-ad9b2b9dfedb" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_5d1a0b75-32f7-4224-b4a4-9de4256787de" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_edc2d85d-afba-4bb5-af8f-6211a9f980a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e4c81c75-e444-4909-824a-ad9b2b9dfedb" xlink:to="loc_us-gaap_RetainedEarningsMember_edc2d85d-afba-4bb5-af8f-6211a9f980a7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_5bb037d9-4ef1-4359-af2f-b1b5a1545167" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_e4c81c75-e444-4909-824a-ad9b2b9dfedb" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_5bb037d9-4ef1-4359-af2f-b1b5a1545167" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_f82e313e-d7eb-4e6f-b8d7-22a3b7824fa3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_ebfd6bbe-35ac-4a29-b7d4-fbb3ce703fb3" xlink:to="loc_us-gaap_StatementClassOfStockAxis_f82e313e-d7eb-4e6f-b8d7-22a3b7824fa3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f82e313e-d7eb-4e6f-b8d7-22a3b7824fa3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f82e313e-d7eb-4e6f-b8d7-22a3b7824fa3" xlink:to="loc_us-gaap_ClassOfStockDomain_f82e313e-d7eb-4e6f-b8d7-22a3b7824fa3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_0652e965-8ef6-4812-abc8-d6147732c337" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f82e313e-d7eb-4e6f-b8d7-22a3b7824fa3" xlink:to="loc_us-gaap_ClassOfStockDomain_0652e965-8ef6-4812-abc8-d6147732c337" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_fc32ef72-bc31-450f-b4e2-76def25c4d7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0652e965-8ef6-4812-abc8-d6147732c337" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_fc32ef72-bc31-450f-b4e2-76def25c4d7a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_9b9ca5e9-095e-4d32-a9e8-09b30c4b0cfa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0652e965-8ef6-4812-abc8-d6147732c337" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_9b9ca5e9-095e-4d32-a9e8-09b30c4b0cfa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConversionOfStockByUniqueDescriptionAxis_019b6e5d-4adf-789a-9a9d-fb130a2488bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockByUniqueDescriptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_ebfd6bbe-35ac-4a29-b7d4-fbb3ce703fb3" xlink:to="loc_us-gaap_ConversionOfStockByUniqueDescriptionAxis_019b6e5d-4adf-789a-9a9d-fb130a2488bb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConversionOfStockNameDomain_019b6e5d-4adf-789a-9a9d-fb130a2488bb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConversionOfStockByUniqueDescriptionAxis_019b6e5d-4adf-789a-9a9d-fb130a2488bb" xlink:to="loc_us-gaap_ConversionOfStockNameDomain_019b6e5d-4adf-789a-9a9d-fb130a2488bb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConversionOfStockNameDomain_019b6e5d-4adf-7f5e-8255-d876e4ac8b82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConversionOfStockByUniqueDescriptionAxis_019b6e5d-4adf-789a-9a9d-fb130a2488bb" xlink:to="loc_us-gaap_ConversionOfStockNameDomain_019b6e5d-4adf-7f5e-8255-d876e4ac8b82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ConversionOfSeriesAPreferredStockToCommonStockMember_019b6e5d-aa2d-7794-8a91-ec4ef03c7112" xlink:href="rnac-20251231.xsd#rnac_ConversionOfSeriesAPreferredStockToCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConversionOfStockNameDomain_019b6e5d-4adf-7f5e-8255-d876e4ac8b82" xlink:to="loc_rnac_ConversionOfSeriesAPreferredStockToCommonStockMember_019b6e5d-aa2d-7794-8a91-ec4ef03c7112" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ConversionOfSeriesBPreferredStockToCommonStockMember_019b6e5d-e830-758e-9148-02df740e264e" xlink:href="rnac-20251231.xsd#rnac_ConversionOfSeriesBPreferredStockToCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConversionOfStockNameDomain_019b6e5d-4adf-7f5e-8255-d876e4ac8b82" xlink:to="loc_rnac_ConversionOfSeriesBPreferredStockToCommonStockMember_019b6e5d-e830-758e-9148-02df740e264e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f2f49297-feda-4f95-aee6-6f0267ab806d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eba0c72a-f6c6-46e8-98f0-2f79c6b85954" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f2f49297-feda-4f95-aee6-6f0267ab806d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_2d5fe946-48c0-4dd1-b291-b5fbea0e1fa3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f2f49297-feda-4f95-aee6-6f0267ab806d" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_2d5fe946-48c0-4dd1-b291-b5fbea0e1fa3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_04c26b15-7b0b-445d-a3b9-f61dfdb3cfee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f2f49297-feda-4f95-aee6-6f0267ab806d" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_04c26b15-7b0b-445d-a3b9-f61dfdb3cfee" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TemporaryEquityStockIssuedDuringPeriodShares_29f25c36-83bb-4c97-857b-163fdcabf40b" xlink:href="rnac-20251231.xsd#rnac_TemporaryEquityStockIssuedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f2f49297-feda-4f95-aee6-6f0267ab806d" xlink:to="loc_rnac_TemporaryEquityStockIssuedDuringPeriodShares_29f25c36-83bb-4c97-857b-163fdcabf40b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_60e413f5-b428-4ded-8a45-427a30e8a20b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f2f49297-feda-4f95-aee6-6f0267ab806d" xlink:to="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_60e413f5-b428-4ded-8a45-427a30e8a20b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TemporaryEquityTransferOfPreferredStockToPermanentEquityShares_3c555c55-1754-46d2-95c7-4070e7d79226" xlink:href="rnac-20251231.xsd#rnac_TemporaryEquityTransferOfPreferredStockToPermanentEquityShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f2f49297-feda-4f95-aee6-6f0267ab806d" xlink:to="loc_rnac_TemporaryEquityTransferOfPreferredStockToPermanentEquityShares_3c555c55-1754-46d2-95c7-4070e7d79226" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TemporaryEquityTransferOfPreferredStockToPermanentEquity_59d5ce39-08cd-4e7f-840f-7c53f7964a8e" xlink:href="rnac-20251231.xsd#rnac_TemporaryEquityTransferOfPreferredStockToPermanentEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f2f49297-feda-4f95-aee6-6f0267ab806d" xlink:to="loc_rnac_TemporaryEquityTransferOfPreferredStockToPermanentEquity_59d5ce39-08cd-4e7f-840f-7c53f7964a8e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_d8fafaed-cb15-4fc6-82a1-a84f8a8c4268" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_6e38ffda-c1da-4f7c-8ca4-2b33c6214e36" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_IncreaseDecreaseInTemporaryEquityOptionRollForwardRollForward_f640688e-9208-4db0-bddb-e3b0e1f6232e" xlink:href="rnac-20251231.xsd#rnac_IncreaseDecreaseInTemporaryEquityOptionRollForwardRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eba0c72a-f6c6-46e8-98f0-2f79c6b85954" xlink:to="loc_rnac_IncreaseDecreaseInTemporaryEquityOptionRollForwardRollForward_f640688e-9208-4db0-bddb-e3b0e1f6232e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TemporaryEquityOptionsCarryingAmountAttributableToParent_f3fa8cf7-3109-470b-9e37-5ccc05d99448" xlink:href="rnac-20251231.xsd#rnac_TemporaryEquityOptionsCarryingAmountAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_IncreaseDecreaseInTemporaryEquityOptionRollForwardRollForward_f640688e-9208-4db0-bddb-e3b0e1f6232e" xlink:to="loc_rnac_TemporaryEquityOptionsCarryingAmountAttributableToParent_f3fa8cf7-3109-470b-9e37-5ccc05d99448" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TemporaryEquityOptionsTransferOfPreferredStockToPermanentEquity_e69c586f-078f-4450-abd1-9f4082862651" xlink:href="rnac-20251231.xsd#rnac_TemporaryEquityOptionsTransferOfPreferredStockToPermanentEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_IncreaseDecreaseInTemporaryEquityOptionRollForwardRollForward_f640688e-9208-4db0-bddb-e3b0e1f6232e" xlink:to="loc_rnac_TemporaryEquityOptionsTransferOfPreferredStockToPermanentEquity_e69c586f-078f-4450-abd1-9f4082862651" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TemporaryEquityOptionsCarryingAmountAttributableToParent_6b866db8-3fa1-4eea-9c5c-f4bc3fa3eac2" xlink:href="rnac-20251231.xsd#rnac_TemporaryEquityOptionsCarryingAmountAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eba0c72a-f6c6-46e8-98f0-2f79c6b85954" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_15d7ef4c-42d4-4b66-a568-cff5599f4076" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_15d7ef4c-42d4-4b66-a568-cff5599f4076" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_5a23735e-ac37-4520-a759-fbb511cdfa97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_StockholdersEquity_5a23735e-ac37-4520-a759-fbb511cdfa97" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_ac55cc33-0b78-4f16-a47b-b9883789231e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_ac55cc33-0b78-4f16-a47b-b9883789231e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TransferOfTemporaryEquityToPermanentEquityShares_425831b4-7de5-4e24-b820-65f86fbc23c8" xlink:href="rnac-20251231.xsd#rnac_TransferOfTemporaryEquityToPermanentEquityShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_rnac_TransferOfTemporaryEquityToPermanentEquityShares_425831b4-7de5-4e24-b820-65f86fbc23c8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TransferOfTemporaryEquityToPermanentEquity_69a7b9bd-aca6-4292-b94c-3a2a1460949f" xlink:href="rnac-20251231.xsd#rnac_TransferOfTemporaryEquityToPermanentEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_rnac_TransferOfTemporaryEquityToPermanentEquity_69a7b9bd-aca6-4292-b94c-3a2a1460949f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_1962311a-c050-440f-bbc5-817f2d08d049" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_1962311a-c050-440f-bbc5-817f2d08d049" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_62ec9393-27af-4ef4-b607-80913cf88563" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_62ec9393-27af-4ef4-b607-80913cf88563" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_82ab64cf-46dd-4c1a-acd8-55e6bad68ccd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_82ab64cf-46dd-4c1a-acd8-55e6bad68ccd" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_cb99a39b-d6a7-4828-93c6-98246daaef9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_cb99a39b-d6a7-4828-93c6-98246daaef9d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_bf9284f0-9e6b-4bf1-a65e-4a6a1b86313a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_bf9284f0-9e6b-4bf1-a65e-4a6a1b86313a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_e21539e9-ab08-4a49-807c-f6661c7460dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_e21539e9-ab08-4a49-807c-f6661c7460dc" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_StockIssuedDuringPeriodSharesCommonWarrantsExercised_019bc65d-3e07-7e16-8a1c-f219b0c68e7a" xlink:href="rnac-20251231.xsd#rnac_StockIssuedDuringPeriodSharesCommonWarrantsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_rnac_StockIssuedDuringPeriodSharesCommonWarrantsExercised_019bc65d-3e07-7e16-8a1c-f219b0c68e7a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_StockIssuedDuringPeriodValueCommonWarrantsExercised_019bc65d-73c8-758e-ba6f-1c4f0c066395" xlink:href="rnac-20251231.xsd#rnac_StockIssuedDuringPeriodValueCommonWarrantsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_rnac_StockIssuedDuringPeriodValueCommonWarrantsExercised_019bc65d-73c8-758e-ba6f-1c4f0c066395" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_2bf06aa7-f13f-459d-a871-7f36a4291e08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_2bf06aa7-f13f-459d-a871-7f36a4291e08" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_bc81bf91-6583-4b54-ac32-37be43152e6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_bc81bf91-6583-4b54-ac32-37be43152e6e" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_88057856-5211-4cb7-8551-ec207cc2302f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_88057856-5211-4cb7-8551-ec207cc2302f" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_0335c19e-bf24-4fe5-8565-d352cfd27f48" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_7f77e501-5e49-445d-8219-dea8b642f21c" xlink:to="loc_us-gaap_NetIncomeLoss_0335c19e-bf24-4fe5-8565-d352cfd27f48" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_460b822e-1707-41b1-af33-8992d7666041" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_f99ad1a1-c26b-4e6a-a318-94259a65c909" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_90f336a4-9b8f-4afb-a4aa-f6dee231fea4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/ConsolidatedStatementsofChangesinConvertiblePreferredStockandStockholdersEquityDeficitParentheticals" xlink:type="simple" xlink:href="rnac-20251231.xsd#ConsolidatedStatementsofChangesinConvertiblePreferredStockandStockholdersEquityDeficitParentheticals"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/ConsolidatedStatementsofChangesinConvertiblePreferredStockandStockholdersEquityDeficitParentheticals" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019bc662-63ac-7594-bcb0-1516c7bda946" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019bc662-63ac-75f7-935e-52539645d091" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019bc662-63ac-7594-bcb0-1516c7bda946" xlink:to="loc_us-gaap_StatementTable_019bc662-63ac-75f7-935e-52539645d091" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019bc662-63ac-7353-bec6-93038fb8622f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019bc662-63ac-75f7-935e-52539645d091" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019bc662-63ac-7353-bec6-93038fb8622f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019bc662-63ac-7353-bec6-93038fb8622f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019bc662-63ac-7353-bec6-93038fb8622f" xlink:to="loc_us-gaap_ClassOfStockDomain_019bc662-63ac-7353-bec6-93038fb8622f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019bc662-63ac-7ad0-8ec6-a5288cd4818f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019bc662-63ac-7353-bec6-93038fb8622f" xlink:to="loc_us-gaap_ClassOfStockDomain_019bc662-63ac-7ad0-8ec6-a5288cd4818f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_019bc662-63ac-72a8-9390-4cfc749c4991" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019bc662-63ac-7ad0-8ec6-a5288cd4818f" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_019bc662-63ac-72a8-9390-4cfc749c4991" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019bc662-63ac-745e-b1a2-fb72cf1a0447" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019bc662-63ac-75f7-935e-52539645d091" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019bc662-63ac-745e-b1a2-fb72cf1a0447" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019bc662-63ac-745e-b1a2-fb72cf1a0447_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019bc662-63ac-745e-b1a2-fb72cf1a0447" xlink:to="loc_us-gaap_EquityComponentDomain_019bc662-63ac-745e-b1a2-fb72cf1a0447_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019bc662-63ac-7638-8822-43d4d97246dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019bc662-63ac-745e-b1a2-fb72cf1a0447" xlink:to="loc_us-gaap_EquityComponentDomain_019bc662-63ac-7638-8822-43d4d97246dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_019bc662-63ac-7b0f-946c-cedf7977856b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019bc662-63ac-7638-8822-43d4d97246dd" xlink:to="loc_us-gaap_PreferredStockMember_019bc662-63ac-7b0f-946c-cedf7977856b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_019bc662-63ac-7211-9583-3e66324210b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019bc662-63ac-7594-bcb0-1516c7bda946" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_019bc662-63ac-7211-9583-3e66324210b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_21967926-7ac8-4de7-b3a4-967a4d6abc72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019bc662-63ac-7594-bcb0-1516c7bda946" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_21967926-7ac8-4de7-b3a4-967a4d6abc72" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/DescriptionoftheBusinessDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#DescriptionoftheBusinessDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/DescriptionoftheBusinessDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019bc592-7689-71b0-bcce-553e3be5cd68" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019bc592-7689-71b0-bcce-553e3be5cd68" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019bc592-7689-7ee3-8a22-d4b937b45528" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019bc592-7689-71b0-bcce-553e3be5cd68" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019bc592-7689-7ee3-8a22-d4b937b45528" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc592-7689-7ee3-8a22-d4b937b45528_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019bc592-7689-7ee3-8a22-d4b937b45528" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc592-7689-7ee3-8a22-d4b937b45528_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc592-7689-774f-8631-362bc82235ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019bc592-7689-7ee3-8a22-d4b937b45528" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc592-7689-774f-8631-362bc82235ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OldCartesianMember_019bc592-7689-7c1b-9191-353795f7acd9" xlink:href="rnac-20251231.xsd#rnac_OldCartesianMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc592-7689-774f-8631-362bc82235ff" xlink:to="loc_rnac_OldCartesianMember_019bc592-7689-7c1b-9191-353795f7acd9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_019bc592-7689-7928-a83c-88bda4bd5648" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityInterestIssuedOrIssuableByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019bc592-7689-71b0-bcce-553e3be5cd68" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_019bc592-7689-7928-a83c-88bda4bd5648" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019bc592-7689-7928-a83c-88bda4bd5648_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_019bc592-7689-7928-a83c-88bda4bd5648" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019bc592-7689-7928-a83c-88bda4bd5648_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019bc592-7689-7a4a-9388-8445e660a72e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_019bc592-7689-7928-a83c-88bda4bd5648" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019bc592-7689-7a4a-9388-8445e660a72e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019bc592-7689-758b-9544-4221ad8b8aac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019bc592-7689-7a4a-9388-8445e660a72e" xlink:to="loc_us-gaap_CommonStockMember_019bc592-7689-758b-9544-4221ad8b8aac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_019bc592-7689-7c35-9e7c-d7d6397ebc63" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_019bc592-7689-7a4a-9388-8445e660a72e" xlink:to="loc_us-gaap_PreferredStockMember_019bc592-7689-7c35-9e7c-d7d6397ebc63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019bc592-7689-7cf5-bbf7-e824505eefd9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019bc592-7689-71b0-bcce-553e3be5cd68" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019bc592-7689-7cf5-bbf7-e824505eefd9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019bc592-7689-7cf5-bbf7-e824505eefd9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019bc592-7689-7cf5-bbf7-e824505eefd9" xlink:to="loc_us-gaap_ClassOfStockDomain_019bc592-7689-7cf5-bbf7-e824505eefd9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019bc592-7689-7b2b-8bba-93ed5dbc9100" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019bc592-7689-7cf5-bbf7-e824505eefd9" xlink:to="loc_us-gaap_ClassOfStockDomain_019bc592-7689-7b2b-8bba-93ed5dbc9100" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_019bc592-7689-7646-bbc8-9ec1be0928b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019bc592-7689-7b2b-8bba-93ed5dbc9100" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_019bc592-7689-7646-bbc8-9ec1be0928b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_019bc592-7689-7152-817a-fb92e35b7e6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_019bc592-7689-7152-817a-fb92e35b7e6e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019bc592-7689-7a47-ba6e-dfd4ea8ab251" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019bc592-7689-7a47-ba6e-dfd4ea8ab251" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityParOrStatedValuePerShare_019bc592-7689-7d5b-97ce-dbc8120a3b07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:to="loc_us-gaap_TemporaryEquityParOrStatedValuePerShare_019bc592-7689-7d5b-97ce-dbc8120a3b07" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019bc594-cd44-7a79-97d6-ac82c279e0f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019bc594-cd44-7a79-97d6-ac82c279e0f6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAndDebtSecuritiesAvailableForSale_019bc594-cd44-7ee0-8543-ac95aa065c3c" xlink:href="rnac-20251231.xsd#rnac_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAndDebtSecuritiesAvailableForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:to="loc_rnac_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAndDebtSecuritiesAvailableForSale_019bc594-cd44-7ee0-8543-ac95aa065c3c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalents_019bc594-cd44-7b67-b236-3b0a9f338dcf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalents_019bc594-cd44-7b67-b236-3b0a9f338dcf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightPercentageOfMilestonePaymentsRoyaltiesAndOtherAmountsToBeDistributed_019bc594-cd44-77df-ab0b-150c8d83fc91" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightPercentageOfMilestonePaymentsRoyaltiesAndOtherAmountsToBeDistributed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019bc592-7689-782e-98d4-6ec8bb82cc1b" xlink:to="loc_rnac_ContingentValueRightPercentageOfMilestonePaymentsRoyaltiesAndOtherAmountsToBeDistributed_019bc594-cd44-77df-ab0b-150c8d83fc91" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#SummaryofSignificantAccountingPoliciesDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProductInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfProductInformationTable_019c2e06-5ecf-774c-87ab-98f46c0da893" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfProductInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:to="loc_us-gaap_ScheduleOfProductInformationTable_019c2e06-5ecf-774c-87ab-98f46c0da893" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019c2e06-a24c-755b-88ad-9cb88c13fae3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfProductInformationTable_019c2e06-5ecf-774c-87ab-98f46c0da893" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019c2e06-a24c-755b-88ad-9cb88c13fae3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c2e06-a24c-755b-88ad-9cb88c13fae3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019c2e06-a24c-755b-88ad-9cb88c13fae3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c2e06-a24c-755b-88ad-9cb88c13fae3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c2e06-a24c-7d45-8c0f-062a18b1d732" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019c2e06-a24c-755b-88ad-9cb88c13fae3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c2e06-a24c-7d45-8c0f-062a18b1d732" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_019c2e06-a24c-7221-bd50-7eb871f83f86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c2e06-a24c-7d45-8c0f-062a18b1d732" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_019c2e06-a24c-7221-bd50-7eb871f83f86" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquipmentMember_019c2e06-a24c-7b42-98f5-b9c36869a741" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c2e06-a24c-7d45-8c0f-062a18b1d732" xlink:to="loc_us-gaap_EquipmentMember_019c2e06-a24c-7b42-98f5-b9c36869a741" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_019c2e06-a24c-76e0-8477-56aacd05935f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c2e06-a24c-7d45-8c0f-062a18b1d732" xlink:to="loc_us-gaap_ComputerEquipmentMember_019c2e06-a24c-76e0-8477-56aacd05935f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c2e07-1275-7e2a-86fe-f9b56f51e31b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfProductInformationTable_019c2e06-5ecf-774c-87ab-98f46c0da893" xlink:to="loc_srt_RangeAxis_019c2e07-1275-7e2a-86fe-f9b56f51e31b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c2e07-1275-7e2a-86fe-f9b56f51e31b_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c2e07-1275-7e2a-86fe-f9b56f51e31b" xlink:to="loc_srt_RangeMember_019c2e07-1275-7e2a-86fe-f9b56f51e31b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c2e07-1275-7d62-9cb1-a29697ca0c83" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c2e07-1275-7e2a-86fe-f9b56f51e31b" xlink:to="loc_srt_RangeMember_019c2e07-1275-7d62-9cb1-a29697ca0c83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c2e07-1275-703f-ae5c-b040d90b0317" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c2e07-1275-7d62-9cb1-a29697ca0c83" xlink:to="loc_srt_MinimumMember_019c2e07-1275-703f-ae5c-b040d90b0317" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c2e07-1275-7e41-989b-56e34d0294d6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c2e07-1275-7d62-9cb1-a29697ca0c83" xlink:to="loc_srt_MaximumMember_019c2e07-1275-7e41-989b-56e34d0294d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019c2e09-0060-7186-bc92-99d9cd5eebd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfProductInformationTable_019c2e06-5ecf-774c-87ab-98f46c0da893" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019c2e09-0060-7186-bc92-99d9cd5eebd4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019c2e09-0060-7186-bc92-99d9cd5eebd4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019c2e09-0060-7186-bc92-99d9cd5eebd4" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019c2e09-0060-7186-bc92-99d9cd5eebd4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019c2e09-0060-7105-bca3-45f1560c849e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019c2e09-0060-7186-bc92-99d9cd5eebd4" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019c2e09-0060-7105-bca3-45f1560c849e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_019c2e09-0060-770f-a357-e0c9840776c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019c2e09-0060-7105-bca3-45f1560c849e" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_019c2e09-0060-770f-a357-e0c9840776c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_019c2e09-5a08-78b9-9b0a-6809e05216a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfProductInformationTable_019c2e06-5ecf-774c-87ab-98f46c0da893" xlink:to="loc_us-gaap_TypeOfArrangementAxis_019c2e09-5a08-78b9-9b0a-6809e05216a4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c2e09-5a08-78b9-9b0a-6809e05216a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019c2e09-5a08-78b9-9b0a-6809e05216a4" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c2e09-5a08-78b9-9b0a-6809e05216a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c2e09-5a08-78f7-b6a9-f799687cd81c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019c2e09-5a08-78b9-9b0a-6809e05216a4" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c2e09-5a08-78f7-b6a9-f799687cd81c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_CyrusBiotechnologyIncMember_019c2e09-5a08-7a9d-9d11-747e74ac4d81" xlink:href="rnac-20251231.xsd#rnac_CyrusBiotechnologyIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c2e09-5a08-78f7-b6a9-f799687cd81c" xlink:to="loc_rnac_CyrusBiotechnologyIncMember_019c2e09-5a08-7a9d-9d11-747e74ac4d81" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_019c2e06-5ecf-7840-8af7-76b0eece0c04" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:to="loc_us-gaap_NumberOfOperatingSegments_019c2e06-5ecf-7840-8af7-76b0eece0c04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019c2e06-a24c-7068-ac68-003eb690e587" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019c2e06-a24c-7068-ac68-003eb690e587" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019c2e07-1275-7fd5-839c-2fd42aa72abc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019c2e07-1275-7fd5-839c-2fd42aa72abc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019c2e07-1275-7b6f-968a-e214283640f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019c2e07-1275-7b6f-968a-e214283640f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_019c2e07-3b4c-7b11-940b-e1afa0a09e78" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_019c2e07-3b4c-7b11-940b-e1afa0a09e78" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_019c8ac6-a842-762d-863b-dace15eba7b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_019c8ac6-a842-762d-863b-dace15eba7b4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetIndefiniteLivedExcludingGoodwillStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_019c9161-92a6-7236-ba9c-23e79ff169c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfIntangibleAssetIndefiniteLivedExcludingGoodwillStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_019c8ac6-a842-762d-863b-dace15eba7b4" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetIndefiniteLivedExcludingGoodwillStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_019c9161-92a6-7236-ba9c-23e79ff169c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment_019c2e09-5a08-7d2f-a51d-39e518beb157" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductInformationLineItems_019c2e06-5ecf-79fd-a834-d7e1d1b6c5e0" xlink:to="loc_us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment_019c2e09-5a08-7d2f-a51d-39e518beb157" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/GoodwillandIntangibleAssetsDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#GoodwillandIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/GoodwillandIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_ff56b4ff-5945-4b30-8dea-21922175a8b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_526a744b-0e67-4bd2-8ee8-36b8a5d1d0fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ff56b4ff-5945-4b30-8dea-21922175a8b3" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_526a744b-0e67-4bd2-8ee8-36b8a5d1d0fb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_e801b2e1-69ef-49cb-a638-72b9135dba4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_526a744b-0e67-4bd2-8ee8-36b8a5d1d0fb" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_e801b2e1-69ef-49cb-a638-72b9135dba4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e801b2e1-69ef-49cb-a638-72b9135dba4a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_e801b2e1-69ef-49cb-a638-72b9135dba4a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e801b2e1-69ef-49cb-a638-72b9135dba4a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_64daf1a3-6d1b-46ac-a51a-fdb64d77c528" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_e801b2e1-69ef-49cb-a638-72b9135dba4a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_64daf1a3-6d1b-46ac-a51a-fdb64d77c528" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OldCartesianMember_9848f39d-1224-4f39-b7a7-cb4f7cc53c7d" xlink:href="rnac-20251231.xsd#rnac_OldCartesianMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_64daf1a3-6d1b-46ac-a51a-fdb64d77c528" xlink:to="loc_rnac_OldCartesianMember_9848f39d-1224-4f39-b7a7-cb4f7cc53c7d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_65fe273d-6891-442b-b5f7-7a8d640f8b8f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_526a744b-0e67-4bd2-8ee8-36b8a5d1d0fb" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_65fe273d-6891-442b-b5f7-7a8d640f8b8f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_65fe273d-6891-442b-b5f7-7a8d640f8b8f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_65fe273d-6891-442b-b5f7-7a8d640f8b8f" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_65fe273d-6891-442b-b5f7-7a8d640f8b8f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_682ba110-4970-478a-848b-e805fd181ece" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_65fe273d-6891-442b-b5f7-7a8d640f8b8f" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_682ba110-4970-478a-848b-e805fd181ece" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_Descartes08ForMGMember_738fe79e-6610-4385-830c-f1914db079ec" xlink:href="rnac-20251231.xsd#rnac_Descartes08ForMGMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_682ba110-4970-478a-848b-e805fd181ece" xlink:to="loc_rnac_Descartes08ForMGMember_738fe79e-6610-4385-830c-f1914db079ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_Descartes08ForSLEMember_7bb15c28-f7ee-42c7-8e8e-5a43aae1fc00" xlink:href="rnac-20251231.xsd#rnac_Descartes08ForSLEMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_682ba110-4970-478a-848b-e805fd181ece" xlink:to="loc_rnac_Descartes08ForSLEMember_7bb15c28-f7ee-42c7-8e8e-5a43aae1fc00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_019c54f1-5495-7773-a646-738ba25e830f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_682ba110-4970-478a-848b-e805fd181ece" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_019c54f1-5495-7773-a646-738ba25e830f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_0633da45-7f86-41e1-9a5e-db96bfeee8ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ff56b4ff-5945-4b30-8dea-21922175a8b3" xlink:to="loc_us-gaap_Goodwill_0633da45-7f86-41e1-9a5e-db96bfeee8ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_d9325d71-4cf8-4f2b-bcbe-97f9cef471dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ff56b4ff-5945-4b30-8dea-21922175a8b3" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_d9325d71-4cf8-4f2b-bcbe-97f9cef471dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_019c54f1-5495-7aad-b464-a29df06e6bac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfIntangibleAssetsFinitelived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ff56b4ff-5945-4b30-8dea-21922175a8b3" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_019c54f1-5495-7aad-b464-a29df06e6bac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_019c553f-037f-72e3-8a04-90af32cc77ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ff56b4ff-5945-4b30-8dea-21922175a8b3" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_019c553f-037f-72e3-8a04-90af32cc77ac" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/InvestmentsDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#InvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/InvestmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_fa929e11-d204-40e6-96b6-c7189df6ca46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_9aeb64e6-3630-40f1-931d-88f84fbb1d47" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_fa929e11-d204-40e6-96b6-c7189df6ca46" xlink:to="loc_us-gaap_MarketableSecuritiesTable_9aeb64e6-3630-40f1-931d-88f84fbb1d47" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_9e011436-1284-465a-bbf6-c33eb0f076f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_9aeb64e6-3630-40f1-931d-88f84fbb1d47" xlink:to="loc_us-gaap_TypeOfArrangementAxis_9e011436-1284-465a-bbf6-c33eb0f076f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_9e011436-1284-465a-bbf6-c33eb0f076f1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_9e011436-1284-465a-bbf6-c33eb0f076f1" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_9e011436-1284-465a-bbf6-c33eb0f076f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_aeae0f19-27af-4c9d-aeab-4a797a4b56ea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_9e011436-1284-465a-bbf6-c33eb0f076f1" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_aeae0f19-27af-4c9d-aeab-4a797a4b56ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_CyrusBiotechnologyIncMember_a7ad1b5d-4110-42bb-982d-6e404c7e589b" xlink:href="rnac-20251231.xsd#rnac_CyrusBiotechnologyIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_aeae0f19-27af-4c9d-aeab-4a797a4b56ea" xlink:to="loc_rnac_CyrusBiotechnologyIncMember_a7ad1b5d-4110-42bb-982d-6e404c7e589b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_cddf4794-e196-4f7f-9249-fda9406986ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_9aeb64e6-3630-40f1-931d-88f84fbb1d47" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_cddf4794-e196-4f7f-9249-fda9406986ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_cddf4794-e196-4f7f-9249-fda9406986ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_cddf4794-e196-4f7f-9249-fda9406986ae" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_cddf4794-e196-4f7f-9249-fda9406986ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_cd9ae7ee-d061-463d-81d8-27dc1f77eab4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_cddf4794-e196-4f7f-9249-fda9406986ae" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_cd9ae7ee-d061-463d-81d8-27dc1f77eab4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SeriesBPreferredStockPurchaseAgreementMember_f3f990e8-5e61-4161-8468-3063593c3384" xlink:href="rnac-20251231.xsd#rnac_SeriesBPreferredStockPurchaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_cd9ae7ee-d061-463d-81d8-27dc1f77eab4" xlink:to="loc_rnac_SeriesBPreferredStockPurchaseAgreementMember_f3f990e8-5e61-4161-8468-3063593c3384" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_EquitySecuritiesStockPurchaseAgreementSharesPurchased_63400e4c-16ab-4aac-b432-205def1c60a3" xlink:href="rnac-20251231.xsd#rnac_EquitySecuritiesStockPurchaseAgreementSharesPurchased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_fa929e11-d204-40e6-96b6-c7189df6ca46" xlink:to="loc_rnac_EquitySecuritiesStockPurchaseAgreementSharesPurchased_63400e4c-16ab-4aac-b432-205def1c60a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_EquitySecuritiesStockPurchaseAgreementParValuePerShare_21660ba9-9ab8-480a-8b6c-78fc54ac8e4f" xlink:href="rnac-20251231.xsd#rnac_EquitySecuritiesStockPurchaseAgreementParValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_fa929e11-d204-40e6-96b6-c7189df6ca46" xlink:to="loc_rnac_EquitySecuritiesStockPurchaseAgreementParValuePerShare_21660ba9-9ab8-480a-8b6c-78fc54ac8e4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_EquitySecuritiesStockPurchaseAgreementPurchasePricePerShare_fda3b00b-d558-4a81-910b-e7a8a5ba1808" xlink:href="rnac-20251231.xsd#rnac_EquitySecuritiesStockPurchaseAgreementPurchasePricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_fa929e11-d204-40e6-96b6-c7189df6ca46" xlink:to="loc_rnac_EquitySecuritiesStockPurchaseAgreementPurchasePricePerShare_fda3b00b-d558-4a81-910b-e7a8a5ba1808" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_83bb7af3-44cb-4873-966e-e9df88367259" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_fa929e11-d204-40e6-96b6-c7189df6ca46" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_83bb7af3-44cb-4873-966e-e9df88367259" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment_019bc665-2a3c-763f-a53b-466121c76542" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_fa929e11-d204-40e6-96b6-c7189df6ca46" xlink:to="loc_us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment_019bc665-2a3c-763f-a53b-466121c76542" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019bc665-7722-7d4f-9174-783d9e5d5060" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_fa929e11-d204-40e6-96b6-c7189df6ca46" xlink:to="loc_us-gaap_EquityMethodInvestments_019bc665-7722-7d4f-9174-783d9e5d5060" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/NetLossPerShareAllocabletoCommonStockholdersScheduleofPotentialDilutiveSharesofCommonStockExcludedfromtheComputationoftheDilutedNetLossPerShareAllocabletoCommonStockholdersDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#NetLossPerShareAllocabletoCommonStockholdersScheduleofPotentialDilutiveSharesofCommonStockExcludedfromtheComputationoftheDilutedNetLossPerShareAllocabletoCommonStockholdersDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/NetLossPerShareAllocabletoCommonStockholdersScheduleofPotentialDilutiveSharesofCommonStockExcludedfromtheComputationoftheDilutedNetLossPerShareAllocabletoCommonStockholdersDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1b817c45-daa2-4c9b-8c5b-eabd9f2e23bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_3a17db65-9c24-47d2-9152-c39e517dc9b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1b817c45-daa2-4c9b-8c5b-eabd9f2e23bf" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_3a17db65-9c24-47d2-9152-c39e517dc9b8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_5d017424-8235-40d9-afcc-3a100257627f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_3a17db65-9c24-47d2-9152-c39e517dc9b8" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_5d017424-8235-40d9-afcc-3a100257627f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_5d017424-8235-40d9-afcc-3a100257627f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_5d017424-8235-40d9-afcc-3a100257627f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_5d017424-8235-40d9-afcc-3a100257627f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_ed0bfc2d-fd7b-40a1-a776-3f4f24ab14b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_5d017424-8235-40d9-afcc-3a100257627f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_ed0bfc2d-fd7b-40a1-a776-3f4f24ab14b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_c188ca9c-2eeb-4f2a-9681-60af4ae6c2e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_ed0bfc2d-fd7b-40a1-a776-3f4f24ab14b7" xlink:to="loc_us-gaap_StockCompensationPlanMember_c188ca9c-2eeb-4f2a-9681-60af4ae6c2e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_67bb6c58-084d-42a9-94c6-34d3590e31ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_ed0bfc2d-fd7b-40a1-a776-3f4f24ab14b7" xlink:to="loc_us-gaap_WarrantMember_67bb6c58-084d-42a9-94c6-34d3590e31ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_7b1b7292-5f00-4916-9861-3d617fd3d96f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_ed0bfc2d-fd7b-40a1-a776-3f4f24ab14b7" xlink:to="loc_us-gaap_PreferredStockMember_7b1b7292-5f00-4916-9861-3d617fd3d96f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_f3b8994a-2568-4bdd-a513-30a942fdb850" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_3a17db65-9c24-47d2-9152-c39e517dc9b8" xlink:to="loc_us-gaap_StatementClassOfStockAxis_f3b8994a-2568-4bdd-a513-30a942fdb850" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f3b8994a-2568-4bdd-a513-30a942fdb850_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f3b8994a-2568-4bdd-a513-30a942fdb850" xlink:to="loc_us-gaap_ClassOfStockDomain_f3b8994a-2568-4bdd-a513-30a942fdb850_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_2814c3d3-48b0-4fff-adc4-8d207d69a696" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f3b8994a-2568-4bdd-a513-30a942fdb850" xlink:to="loc_us-gaap_ClassOfStockDomain_2814c3d3-48b0-4fff-adc4-8d207d69a696" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_5600d967-6e63-4777-bf5a-d47172401cd1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_2814c3d3-48b0-4fff-adc4-8d207d69a696" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_5600d967-6e63-4777-bf5a-d47172401cd1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_74bec624-4b72-4665-bd0e-6213adcbcc16" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_2814c3d3-48b0-4fff-adc4-8d207d69a696" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_74bec624-4b72-4665-bd0e-6213adcbcc16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_ed67bb46-f073-44b4-89ef-9825146c8f7e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1b817c45-daa2-4c9b-8c5b-eabd9f2e23bf" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_ed67bb46-f073-44b4-89ef-9825146c8f7e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsScheduleofAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#FairValueMeasurementsScheduleofAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsScheduleofAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_cb8210ed-2ea3-443f-80d9-5130ce6fc58d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_9aeb4117-8e1a-466f-a7a2-2a99bc4d6c46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_cb8210ed-2ea3-443f-80d9-5130ce6fc58d" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_9aeb4117-8e1a-466f-a7a2-2a99bc4d6c46" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_dcbf623e-2c4c-4303-8333-9e23cce461db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_9aeb4117-8e1a-466f-a7a2-2a99bc4d6c46" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_dcbf623e-2c4c-4303-8333-9e23cce461db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_dcbf623e-2c4c-4303-8333-9e23cce461db_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_dcbf623e-2c4c-4303-8333-9e23cce461db" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_dcbf623e-2c4c-4303-8333-9e23cce461db_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_a20877de-4b96-4997-bc3f-21ca55e36889" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_dcbf623e-2c4c-4303-8333-9e23cce461db" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_a20877de-4b96-4997-bc3f-21ca55e36889" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_26084725-606c-4b9f-9595-7634bf90c82b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_a20877de-4b96-4997-bc3f-21ca55e36889" xlink:to="loc_us-gaap_MoneyMarketFundsMember_26084725-606c-4b9f-9595-7634bf90c82b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6f828ad5-8777-48ac-9928-043b348763ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_9aeb4117-8e1a-466f-a7a2-2a99bc4d6c46" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6f828ad5-8777-48ac-9928-043b348763ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6f828ad5-8777-48ac-9928-043b348763ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6f828ad5-8777-48ac-9928-043b348763ae" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6f828ad5-8777-48ac-9928-043b348763ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c8e7e391-6d93-4ef3-9af5-7532c9eda6ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6f828ad5-8777-48ac-9928-043b348763ae" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c8e7e391-6d93-4ef3-9af5-7532c9eda6ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_97de93d6-0fac-4815-9ded-cc57f73563e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c8e7e391-6d93-4ef3-9af5-7532c9eda6ae" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_97de93d6-0fac-4815-9ded-cc57f73563e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_7280f447-aa3b-4dbd-a027-df1f5c9ead98" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c8e7e391-6d93-4ef3-9af5-7532c9eda6ae" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_7280f447-aa3b-4dbd-a027-df1f5c9ead98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_0f3cd02a-e805-4e3c-98f7-9f6cdda180a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c8e7e391-6d93-4ef3-9af5-7532c9eda6ae" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_0f3cd02a-e805-4e3c-98f7-9f6cdda180a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_8b819317-179d-4144-a7fa-deb756760275" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_cb8210ed-2ea3-443f-80d9-5130ce6fc58d" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_8b819317-179d-4144-a7fa-deb756760275" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_6ad1015c-2903-4f5c-ab56-6b617a6eae71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_8b819317-179d-4144-a7fa-deb756760275" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_6ad1015c-2903-4f5c-ab56-6b617a6eae71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_db08e015-71e5-49a0-82d7-c4c4a19336d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_8b819317-179d-4144-a7fa-deb756760275" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_db08e015-71e5-49a0-82d7-c4c4a19336d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_c9356f82-2dd7-4df4-882d-d232c09e8346" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_cb8210ed-2ea3-443f-80d9-5130ce6fc58d" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_c9356f82-2dd7-4df4-882d-d232c09e8346" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_58ee6b25-8e45-489c-a738-adc9ee3bbd1b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_c9356f82-2dd7-4df4-882d-d232c09e8346" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_58ee6b25-8e45-489c-a738-adc9ee3bbd1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightLiability_5a262da6-8061-43a0-8c83-b9511c1b0dab" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_c9356f82-2dd7-4df4-882d-d232c09e8346" xlink:to="loc_rnac_ContingentValueRightLiability_5a262da6-8061-43a0-8c83-b9511c1b0dab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_050d3f02-135c-4c43-ae05-d7344d08ff96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_c9356f82-2dd7-4df4-882d-d232c09e8346" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_050d3f02-135c-4c43-ae05-d7344d08ff96" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#FairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_53768341-963e-4df6-94f1-de1920b9dc71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_53768341-963e-4df6-94f1-de1920b9dc71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_53768341-963e-4df6-94f1-de1920b9dc71_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_53768341-963e-4df6-94f1-de1920b9dc71" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_53768341-963e-4df6-94f1-de1920b9dc71_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_9689a89d-2e64-40d1-8103-36698148fb37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_53768341-963e-4df6-94f1-de1920b9dc71" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_9689a89d-2e64-40d1-8103-36698148fb37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_b35379fe-90bd-460f-98c3-a839d50efc23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_9689a89d-2e64-40d1-8103-36698148fb37" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_b35379fe-90bd-460f-98c3-a839d50efc23" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueAxis_7296d30c-0c68-4cb5-a48b-94b4bf0b2eaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:to="loc_us-gaap_ValuationTechniqueAxis_7296d30c-0c68-4cb5-a48b-94b4bf0b2eaf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_7296d30c-0c68-4cb5-a48b-94b4bf0b2eaf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationTechniqueAxis_7296d30c-0c68-4cb5-a48b-94b4bf0b2eaf" xlink:to="loc_us-gaap_ValuationTechniqueDomain_7296d30c-0c68-4cb5-a48b-94b4bf0b2eaf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_1d3ff674-41b0-4e38-8d02-bb1f2728ebb0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationTechniqueAxis_7296d30c-0c68-4cb5-a48b-94b4bf0b2eaf" xlink:to="loc_us-gaap_ValuationTechniqueDomain_1d3ff674-41b0-4e38-8d02-bb1f2728ebb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueOptionPricingModelMember_d4a6c2a0-ae15-4131-a91e-fc90d17eb2ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueOptionPricingModelMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_1d3ff674-41b0-4e38-8d02-bb1f2728ebb0" xlink:to="loc_us-gaap_ValuationTechniqueOptionPricingModelMember_d4a6c2a0-ae15-4131-a91e-fc90d17eb2ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_4fefcff9-d428-4ab8-9301-5e3e7d6c62aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_4fefcff9-d428-4ab8-9301-5e3e7d6c62aa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_4fefcff9-d428-4ab8-9301-5e3e7d6c62aa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_4fefcff9-d428-4ab8-9301-5e3e7d6c62aa" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_4fefcff9-d428-4ab8-9301-5e3e7d6c62aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a7b30350-c05c-4d12-b766-c511c265b72b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_4fefcff9-d428-4ab8-9301-5e3e7d6c62aa" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a7b30350-c05c-4d12-b766-c511c265b72b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OldCartesianMember_f27a411a-64ea-40ff-ad2b-13e6943831fe" xlink:href="rnac-20251231.xsd#rnac_OldCartesianMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_a7b30350-c05c-4d12-b766-c511c265b72b" xlink:to="loc_rnac_OldCartesianMember_f27a411a-64ea-40ff-ad2b-13e6943831fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_d21844a8-1ee7-4132-a7e9-f76507c66cbb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_d21844a8-1ee7-4132-a7e9-f76507c66cbb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_d21844a8-1ee7-4132-a7e9-f76507c66cbb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_d21844a8-1ee7-4132-a7e9-f76507c66cbb" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_d21844a8-1ee7-4132-a7e9-f76507c66cbb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_fc5ad822-d28b-4c08-adf4-142056dccade" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_d21844a8-1ee7-4132-a7e9-f76507c66cbb" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_fc5ad822-d28b-4c08-adf4-142056dccade" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_A2022WarrantsMember_6a2d9b5d-aa24-4c77-b3e8-20806b0ab7c4" xlink:href="rnac-20251231.xsd#rnac_A2022WarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_fc5ad822-d28b-4c08-adf4-142056dccade" xlink:to="loc_rnac_A2022WarrantsMember_6a2d9b5d-aa24-4c77-b3e8-20806b0ab7c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_759f2da4-688a-46ed-8825-2ec625903865" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:to="loc_us-gaap_FinancialInstrumentAxis_759f2da4-688a-46ed-8825-2ec625903865" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_759f2da4-688a-46ed-8825-2ec625903865_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_759f2da4-688a-46ed-8825-2ec625903865" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_759f2da4-688a-46ed-8825-2ec625903865_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1e288a54-f53f-4c4e-92fd-009ce9eecad8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_759f2da4-688a-46ed-8825-2ec625903865" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1e288a54-f53f-4c4e-92fd-009ce9eecad8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightMember_137b694e-639c-4d96-abcc-48593a78e251" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1e288a54-f53f-4c4e-92fd-009ce9eecad8" xlink:to="loc_rnac_ContingentValueRightMember_137b694e-639c-4d96-abcc-48593a78e251" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_4829cf99-9c72-444e-99c6-670b6c8c1d0c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:to="loc_srt_CounterpartyNameAxis_4829cf99-9c72-444e-99c6-670b6c8c1d0c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_4829cf99-9c72-444e-99c6-670b6c8c1d0c_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_4829cf99-9c72-444e-99c6-670b6c8c1d0c" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_4829cf99-9c72-444e-99c6-670b6c8c1d0c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_d31e9703-a81d-444e-bff6-b7180679233a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_4829cf99-9c72-444e-99c6-670b6c8c1d0c" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_d31e9703-a81d-444e-bff6-b7180679233a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SwedishOrphanBiovitrumABSOBIMember_8e02dac9-8617-403b-b558-5f9c67abaa1f" xlink:href="rnac-20251231.xsd#rnac_SwedishOrphanBiovitrumABSOBIMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_d31e9703-a81d-444e-bff6-b7180679233a" xlink:to="loc_rnac_SwedishOrphanBiovitrumABSOBIMember_8e02dac9-8617-403b-b558-5f9c67abaa1f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_13b3d42e-5abe-4007-877a-96359b3b4d48" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:to="loc_srt_ProductOrServiceAxis_13b3d42e-5abe-4007-877a-96359b3b4d48" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_13b3d42e-5abe-4007-877a-96359b3b4d48_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_13b3d42e-5abe-4007-877a-96359b3b4d48" xlink:to="loc_srt_ProductsAndServicesDomain_13b3d42e-5abe-4007-877a-96359b3b4d48_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_b15ee99b-765e-4173-8d94-62045b93e545" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_13b3d42e-5abe-4007-877a-96359b3b4d48" xlink:to="loc_srt_ProductsAndServicesDomain_b15ee99b-765e-4173-8d94-62045b93e545" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LicenseMember_98b4eba7-6739-4397-8fff-f627fa4a8148" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LicenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_b15ee99b-765e-4173-8d94-62045b93e545" xlink:to="loc_us-gaap_LicenseMember_98b4eba7-6739-4397-8fff-f627fa4a8148" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_9c2a2e36-7491-482d-b1da-157571aa2ba1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_4e8223d9-5acb-4b3d-a1f2-65c2f27347a8" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_9c2a2e36-7491-482d-b1da-157571aa2ba1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9c2a2e36-7491-482d-b1da-157571aa2ba1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_9c2a2e36-7491-482d-b1da-157571aa2ba1" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9c2a2e36-7491-482d-b1da-157571aa2ba1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_26af001c-d00d-4f37-8e99-4c4cfc5fedc3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_9c2a2e36-7491-482d-b1da-157571aa2ba1" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_26af001c-d00d-4f37-8e99-4c4cfc5fedc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SecuritiesPurchaseAgreement2023Member_38bb4db5-ee70-4240-9dfb-7477d91de223" xlink:href="rnac-20251231.xsd#rnac_SecuritiesPurchaseAgreement2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_26af001c-d00d-4f37-8e99-4c4cfc5fedc3" xlink:to="loc_rnac_SecuritiesPurchaseAgreement2023Member_38bb4db5-ee70-4240-9dfb-7477d91de223" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet_ed3f4539-a285-4851-8b2d-3e1d08db13f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet_ed3f4539-a285-4851-8b2d-3e1d08db13f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet_e72fee84-adc0-4b4b-b748-cca268bf2124" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet_e72fee84-adc0-4b4b-b748-cca268bf2124" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_ff94423e-5265-47fe-a389-97fa9d25d9e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_ff94423e-5265-47fe-a389-97fa9d25d9e8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight_07b91f51-0679-437a-9c81-061e2ab480cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight_07b91f51-0679-437a-9c81-061e2ab480cc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightPercentageOfMilestonePaymentsRoyaltiesAndOtherAmountsToBeDistributed_f1123a34-60c1-4efe-9922-d56e8c148743" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightPercentageOfMilestonePaymentsRoyaltiesAndOtherAmountsToBeDistributed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_rnac_ContingentValueRightPercentageOfMilestonePaymentsRoyaltiesAndOtherAmountsToBeDistributed_f1123a34-60c1-4efe-9922-d56e8c148743" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightLiabilityDeductionFromDistributionGeneralAndAdministrativeOverhead_5399bec7-b64a-46c5-9b36-e9c636ff069a" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightLiabilityDeductionFromDistributionGeneralAndAdministrativeOverhead"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_rnac_ContingentValueRightLiabilityDeductionFromDistributionGeneralAndAdministrativeOverhead_5399bec7-b64a-46c5-9b36-e9c636ff069a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_7c1baf9d-a141-4191-a34a-1ef5ead44f04" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_us-gaap_ContractWithCustomerLiability_7c1baf9d-a141-4191-a34a-1ef5ead44f04" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_a96cf9a0-e246-4dc3-931a-9bffe2e10522" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_a96cf9a0-e246-4dc3-931a-9bffe2e10522" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SettlementOfDerivativeLiabilityShares_9ea02ab7-9acc-4ced-9773-ba42ae2c70b6" xlink:href="rnac-20251231.xsd#rnac_SettlementOfDerivativeLiabilityShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_rnac_SettlementOfDerivativeLiabilityShares_9ea02ab7-9acc-4ced-9773-ba42ae2c70b6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_IssuanceOfStockForSettlementOfForwardContractLiability_dd21bd63-b07c-499e-9ac7-a7895dc7240d" xlink:href="rnac-20251231.xsd#rnac_IssuanceOfStockForSettlementOfForwardContractLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_af9d3b19-2777-4c95-92f2-b85bf2ebaa90" xlink:to="loc_rnac_IssuanceOfStockForSettlementOfForwardContractLiability_dd21bd63-b07c-499e-9ac7-a7895dc7240d" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesforWarrantsLiabilityDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesforWarrantsLiabilityDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesforWarrantsLiabilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_2ae4bc70-6e2d-4771-8056-1c761fb73e18" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_75038756-fc34-4a88-adea-f12cf279fb77" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_2ae4bc70-6e2d-4771-8056-1c761fb73e18" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_75038756-fc34-4a88-adea-f12cf279fb77" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueAxis_954f5695-2954-4d3f-a2fd-a7622ec622e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_75038756-fc34-4a88-adea-f12cf279fb77" xlink:to="loc_us-gaap_ValuationTechniqueAxis_954f5695-2954-4d3f-a2fd-a7622ec622e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_954f5695-2954-4d3f-a2fd-a7622ec622e1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationTechniqueAxis_954f5695-2954-4d3f-a2fd-a7622ec622e1" xlink:to="loc_us-gaap_ValuationTechniqueDomain_954f5695-2954-4d3f-a2fd-a7622ec622e1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_69a0bff2-8ca1-4a37-8e1d-f5397ce6e84e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationTechniqueAxis_954f5695-2954-4d3f-a2fd-a7622ec622e1" xlink:to="loc_us-gaap_ValuationTechniqueDomain_69a0bff2-8ca1-4a37-8e1d-f5397ce6e84e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueOptionPricingModelMember_64f77a6f-33e9-4b72-b4b8-f187cc6dc61c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueOptionPricingModelMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_69a0bff2-8ca1-4a37-8e1d-f5397ce6e84e" xlink:to="loc_us-gaap_ValuationTechniqueOptionPricingModelMember_64f77a6f-33e9-4b72-b4b8-f187cc6dc61c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_b33f2d93-4d3c-459f-a030-7e6b6fe186af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_75038756-fc34-4a88-adea-f12cf279fb77" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_b33f2d93-4d3c-459f-a030-7e6b6fe186af" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_b33f2d93-4d3c-459f-a030-7e6b6fe186af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_b33f2d93-4d3c-459f-a030-7e6b6fe186af" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_b33f2d93-4d3c-459f-a030-7e6b6fe186af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_f3789880-e359-4937-948a-aa89e779866d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_b33f2d93-4d3c-459f-a030-7e6b6fe186af" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_f3789880-e359-4937-948a-aa89e779866d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_40781729-e69b-4500-8809-51710a9b4228" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_f3789880-e359-4937-948a-aa89e779866d" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_40781729-e69b-4500-8809-51710a9b4228" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_224d4d4c-8615-43a0-a8c0-9f503f3622e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_f3789880-e359-4937-948a-aa89e779866d" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_224d4d4c-8615-43a0-a8c0-9f503f3622e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_087c0431-af9f-4201-94b9-526fc64b6cf6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExpectedTermMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_f3789880-e359-4937-948a-aa89e779866d" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_087c0431-af9f-4201-94b9-526fc64b6cf6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_0c090242-67a1-4352-9edf-94ef3ddcb628" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputPriceVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_f3789880-e359-4937-948a-aa89e779866d" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_0c090242-67a1-4352-9edf-94ef3ddcb628" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_3aaaf719-04d9-497e-9baa-9b425c983080" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_75038756-fc34-4a88-adea-f12cf279fb77" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_3aaaf719-04d9-497e-9baa-9b425c983080" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_3aaaf719-04d9-497e-9baa-9b425c983080_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_3aaaf719-04d9-497e-9baa-9b425c983080" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_3aaaf719-04d9-497e-9baa-9b425c983080_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_30546fee-7311-4a22-83b7-144ae5ee1469" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_3aaaf719-04d9-497e-9baa-9b425c983080" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_30546fee-7311-4a22-83b7-144ae5ee1469" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_A2022WarrantsMember_b452015c-4036-401a-8b35-5e96d0e4ba8a" xlink:href="rnac-20251231.xsd#rnac_A2022WarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_30546fee-7311-4a22-83b7-144ae5ee1469" xlink:to="loc_rnac_A2022WarrantsMember_b452015c-4036-401a-8b35-5e96d0e4ba8a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a04c1bb1-7f69-4add-bc5f-f029d42cff58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_75038756-fc34-4a88-adea-f12cf279fb77" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a04c1bb1-7f69-4add-bc5f-f029d42cff58" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a04c1bb1-7f69-4add-bc5f-f029d42cff58_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a04c1bb1-7f69-4add-bc5f-f029d42cff58" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a04c1bb1-7f69-4add-bc5f-f029d42cff58_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3ec2e802-e25f-488c-bb69-3718310f39c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a04c1bb1-7f69-4add-bc5f-f029d42cff58" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3ec2e802-e25f-488c-bb69-3718310f39c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_20123d60-cea0-43a5-9981-276fa41864ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3ec2e802-e25f-488c-bb69-3718310f39c4" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_20123d60-cea0-43a5-9981-276fa41864ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_44d488be-bc75-4de7-a98b-6560afbda956" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_2ae4bc70-6e2d-4771-8056-1c761fb73e18" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_44d488be-bc75-4de7-a98b-6560afbda956" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_0c6f03be-6b15-49f3-bfb8-23ede3200691" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_2ae4bc70-6e2d-4771-8056-1c761fb73e18" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_0c6f03be-6b15-49f3-bfb8-23ede3200691" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsScheduleofRollForwardofFairValuefortheCompanysLevel3WarrantLiabilitiesandCVRLiabilityDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#FairValueMeasurementsScheduleofRollForwardofFairValuefortheCompanysLevel3WarrantLiabilitiesandCVRLiabilityDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsScheduleofRollForwardofFairValuefortheCompanysLevel3WarrantLiabilitiesandCVRLiabilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_5c05441a-1c34-4eec-808c-e94bcc132050" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_fff39e2c-d7e5-41c2-a99b-082e1b404e1d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_5c05441a-1c34-4eec-808c-e94bcc132050" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_fff39e2c-d7e5-41c2-a99b-082e1b404e1d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_f8423d07-9bd1-465d-9a83-cdf5b9ab2895" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_fff39e2c-d7e5-41c2-a99b-082e1b404e1d" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_f8423d07-9bd1-465d-9a83-cdf5b9ab2895" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_f8423d07-9bd1-465d-9a83-cdf5b9ab2895_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_f8423d07-9bd1-465d-9a83-cdf5b9ab2895" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_f8423d07-9bd1-465d-9a83-cdf5b9ab2895_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_93784ca4-dda8-43bb-bede-d7fd64cef40e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_f8423d07-9bd1-465d-9a83-cdf5b9ab2895" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_93784ca4-dda8-43bb-bede-d7fd64cef40e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_3553cccd-80e6-4177-a863-22efb223888e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_93784ca4-dda8-43bb-bede-d7fd64cef40e" xlink:to="loc_us-gaap_WarrantMember_3553cccd-80e6-4177-a863-22efb223888e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightMember_e51aa7d9-3aa6-4846-ad83-af36361f9225" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_93784ca4-dda8-43bb-bede-d7fd64cef40e" xlink:to="loc_rnac_ContingentValueRightMember_e51aa7d9-3aa6-4846-ad83-af36361f9225" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_06dd018f-6586-4e09-b5f5-f38aa587a3b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_5c05441a-1c34-4eec-808c-e94bcc132050" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_06dd018f-6586-4e09-b5f5-f38aa587a3b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_3803a2d1-5f49-4479-b4b2-c061518b2be5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_06dd018f-6586-4e09-b5f5-f38aa587a3b4" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_3803a2d1-5f49-4479-b4b2-c061518b2be5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_019be9df-e6a9-7a47-bf63-f3051b7fe873" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_06dd018f-6586-4e09-b5f5-f38aa587a3b4" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_019be9df-e6a9-7a47-bf63-f3051b7fe873" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_879e3a1f-4eaf-458c-ab37-1738a131f0e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_06dd018f-6586-4e09-b5f5-f38aa587a3b4" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_879e3a1f-4eaf-458c-ab37-1738a131f0e0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_928ea624-62a7-428a-a8d9-d63a8f7f7187" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesforCVRLiabilityDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesforCVRLiabilityDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesforCVRLiabilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_06f5c6a9-3d32-40ca-bdd1-417737857fd9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_320a8c2b-a314-45c0-a788-4af5d4362a70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_06f5c6a9-3d32-40ca-bdd1-417737857fd9" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_320a8c2b-a314-45c0-a788-4af5d4362a70" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_b0a51b1c-461a-4b63-a8e7-db78449f8127" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_320a8c2b-a314-45c0-a788-4af5d4362a70" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_b0a51b1c-461a-4b63-a8e7-db78449f8127" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_b0a51b1c-461a-4b63-a8e7-db78449f8127_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_b0a51b1c-461a-4b63-a8e7-db78449f8127" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_b0a51b1c-461a-4b63-a8e7-db78449f8127_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a849f56c-533a-46a3-9d31-e645f6fec82d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_b0a51b1c-461a-4b63-a8e7-db78449f8127" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a849f56c-533a-46a3-9d31-e645f6fec82d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_9ebab961-33b1-4880-9d46-0b5426d905ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a849f56c-533a-46a3-9d31-e645f6fec82d" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_9ebab961-33b1-4880-9d46-0b5426d905ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_1b0e8931-348e-4e89-83f7-a12ad7e6706a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_320a8c2b-a314-45c0-a788-4af5d4362a70" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_1b0e8931-348e-4e89-83f7-a12ad7e6706a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_1b0e8931-348e-4e89-83f7-a12ad7e6706a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_1b0e8931-348e-4e89-83f7-a12ad7e6706a" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_1b0e8931-348e-4e89-83f7-a12ad7e6706a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_181b4ad3-9dae-4e91-9a06-2a0ff00fb201" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_1b0e8931-348e-4e89-83f7-a12ad7e6706a" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_181b4ad3-9dae-4e91-9a06-2a0ff00fb201" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ProbabilityOfSuccessMember_9b031fee-14dd-4efa-bfad-591feb0fa0af" xlink:href="rnac-20251231.xsd#rnac_ProbabilityOfSuccessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_181b4ad3-9dae-4e91-9a06-2a0ff00fb201" xlink:to="loc_rnac_ProbabilityOfSuccessMember_9b031fee-14dd-4efa-bfad-591feb0fa0af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_MeasurementInputRevenueVolatilityMember_c30b7085-501f-45bf-b5c8-8673db88e228" xlink:href="rnac-20251231.xsd#rnac_MeasurementInputRevenueVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_181b4ad3-9dae-4e91-9a06-2a0ff00fb201" xlink:to="loc_rnac_MeasurementInputRevenueVolatilityMember_c30b7085-501f-45bf-b5c8-8673db88e228" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_8ba33d14-242c-44ed-bc24-0f16647a1eb9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_320a8c2b-a314-45c0-a788-4af5d4362a70" xlink:to="loc_srt_RangeAxis_8ba33d14-242c-44ed-bc24-0f16647a1eb9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_8ba33d14-242c-44ed-bc24-0f16647a1eb9_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_8ba33d14-242c-44ed-bc24-0f16647a1eb9" xlink:to="loc_srt_RangeMember_8ba33d14-242c-44ed-bc24-0f16647a1eb9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_683e45af-82d8-4e70-9630-be5f9b8b7cf4" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_8ba33d14-242c-44ed-bc24-0f16647a1eb9" xlink:to="loc_srt_RangeMember_683e45af-82d8-4e70-9630-be5f9b8b7cf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_24bbeb3a-cb6a-42b1-86f0-b66e67aa1151" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_683e45af-82d8-4e70-9630-be5f9b8b7cf4" xlink:to="loc_srt_MinimumMember_24bbeb3a-cb6a-42b1-86f0-b66e67aa1151" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_fb0d012c-cc28-4587-9191-1e097733c366" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_683e45af-82d8-4e70-9630-be5f9b8b7cf4" xlink:to="loc_srt_MaximumMember_fb0d012c-cc28-4587-9191-1e097733c366" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ContingentValueRightLiabilityMeasurementInput_ee94becb-8743-4c2b-968a-0a1a296ad719" xlink:href="rnac-20251231.xsd#rnac_ContingentValueRightLiabilityMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_06f5c6a9-3d32-40ca-bdd1-417737857fd9" xlink:to="loc_rnac_ContingentValueRightLiabilityMeasurementInput_ee94becb-8743-4c2b-968a-0a1a296ad719" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/PropertyandEquipmentScheduleofPropertyandEquipmentDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#PropertyandEquipmentScheduleofPropertyandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/PropertyandEquipmentScheduleofPropertyandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b015cc4f-7b44-4ada-84bf-c172017c4230" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_9d188825-d78b-47e2-8574-8a070115142f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b015cc4f-7b44-4ada-84bf-c172017c4230" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_9d188825-d78b-47e2-8574-8a070115142f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_3d5eac29-2db9-45df-a0e7-d0674f66928b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_9d188825-d78b-47e2-8574-8a070115142f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_3d5eac29-2db9-45df-a0e7-d0674f66928b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3d5eac29-2db9-45df-a0e7-d0674f66928b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_3d5eac29-2db9-45df-a0e7-d0674f66928b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3d5eac29-2db9-45df-a0e7-d0674f66928b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_67ee2565-d105-4250-a10d-fa3538b8f229" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_3d5eac29-2db9-45df-a0e7-d0674f66928b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_67ee2565-d105-4250-a10d-fa3538b8f229" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LaboratoryEquipmentMember_85cecedb-1e56-409b-b7eb-35b3cf49f8cb" xlink:href="rnac-20251231.xsd#rnac_LaboratoryEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_67ee2565-d105-4250-a10d-fa3538b8f229" xlink:to="loc_rnac_LaboratoryEquipmentMember_85cecedb-1e56-409b-b7eb-35b3cf49f8cb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_c1532059-cd29-41fe-8ae8-042290b0a9f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_67ee2565-d105-4250-a10d-fa3538b8f229" xlink:to="loc_us-gaap_ComputerEquipmentMember_c1532059-cd29-41fe-8ae8-042290b0a9f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_f0e9edda-7ac8-4b23-8e82-675ac1a00960" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_67ee2565-d105-4250-a10d-fa3538b8f229" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_f0e9edda-7ac8-4b23-8e82-675ac1a00960" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_216e095f-2bb8-4015-8756-70e8b64341e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_67ee2565-d105-4250-a10d-fa3538b8f229" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_216e095f-2bb8-4015-8756-70e8b64341e1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OfficeEquipmentMember_5a65301c-9db5-4f6f-9011-0b58242f6216" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OfficeEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_67ee2565-d105-4250-a10d-fa3538b8f229" xlink:to="loc_us-gaap_OfficeEquipmentMember_5a65301c-9db5-4f6f-9011-0b58242f6216" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_97c2b045-d803-4165-83cb-e63b5e56012f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_67ee2565-d105-4250-a10d-fa3538b8f229" xlink:to="loc_us-gaap_ConstructionInProgressMember_97c2b045-d803-4165-83cb-e63b5e56012f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_5f6f12a5-0cae-497c-8091-478c8c7f877d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b015cc4f-7b44-4ada-84bf-c172017c4230" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_5f6f12a5-0cae-497c-8091-478c8c7f877d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_43878798-bf6e-4b40-a9e5-b6cdb7ecfb1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b015cc4f-7b44-4ada-84bf-c172017c4230" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_43878798-bf6e-4b40-a9e5-b6cdb7ecfb1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_142e1958-04d9-4dd2-bc7f-b6ad18e6aca7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b015cc4f-7b44-4ada-84bf-c172017c4230" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_142e1958-04d9-4dd2-bc7f-b6ad18e6aca7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/Leases7495NewHorizonWayLeasesNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#Leases7495NewHorizonWayLeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/Leases7495NewHorizonWayLeasesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeasedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_5d05be07-0348-44aa-935a-ee5d0e04db87" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfOperatingLeasedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_5d05be07-0348-44aa-935a-ee5d0e04db87" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementAxis_10f18f1a-0608-4a09-aecd-4ba12799e1cd" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_5d05be07-0348-44aa-935a-ee5d0e04db87" xlink:to="loc_rnac_LeaseArrangementAxis_10f18f1a-0608-4a09-aecd-4ba12799e1cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementDomain_10f18f1a-0608-4a09-aecd-4ba12799e1cd_default" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_rnac_LeaseArrangementAxis_10f18f1a-0608-4a09-aecd-4ba12799e1cd" xlink:to="loc_rnac_LeaseArrangementDomain_10f18f1a-0608-4a09-aecd-4ba12799e1cd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementDomain_93b968ea-a110-4315-ab2c-6fe8fed7c757" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_rnac_LeaseArrangementAxis_10f18f1a-0608-4a09-aecd-4ba12799e1cd" xlink:to="loc_rnac_LeaseArrangementDomain_93b968ea-a110-4315-ab2c-6fe8fed7c757" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_A7495NewHorizonWayLeaseMember_7b058c1e-b711-4c28-88be-bc2c06910651" xlink:href="rnac-20251231.xsd#rnac_A7495NewHorizonWayLeaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_LeaseArrangementDomain_93b968ea-a110-4315-ab2c-6fe8fed7c757" xlink:to="loc_rnac_A7495NewHorizonWayLeaseMember_7b058c1e-b711-4c28-88be-bc2c06910651" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_927cc8af-135b-4274-9ec3-0aff0c576cad" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_5d05be07-0348-44aa-935a-ee5d0e04db87" xlink:to="loc_srt_RangeAxis_927cc8af-135b-4274-9ec3-0aff0c576cad" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_927cc8af-135b-4274-9ec3-0aff0c576cad_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_927cc8af-135b-4274-9ec3-0aff0c576cad" xlink:to="loc_srt_RangeMember_927cc8af-135b-4274-9ec3-0aff0c576cad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_7fd113ef-682f-4a99-b9cb-5450e9f4a3fd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_927cc8af-135b-4274-9ec3-0aff0c576cad" xlink:to="loc_srt_RangeMember_7fd113ef-682f-4a99-b9cb-5450e9f4a3fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_256a57f3-7c30-4d0f-98c3-d873cc7b6ae9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_7fd113ef-682f-4a99-b9cb-5450e9f4a3fd" xlink:to="loc_srt_MinimumMember_256a57f3-7c30-4d0f-98c3-d873cc7b6ae9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_919e34bd-3209-4e15-9126-7cb3f616ea6d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_7fd113ef-682f-4a99-b9cb-5450e9f4a3fd" xlink:to="loc_srt_MaximumMember_919e34bd-3209-4e15-9126-7cb3f616ea6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_7654a64c-aa37-4ec0-9270-bc2a011ea104" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AreaOfRealEstateProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_7654a64c-aa37-4ec0-9270-bc2a011ea104" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_6c5224f0-dbc4-4b8e-8c01-daf3d59d97d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_6c5224f0-dbc4-4b8e-8c01-daf3d59d97d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LesseeOperatingLeaseNumberOfOptionsToExtend_8d2c7153-7ff8-4e3f-9c41-2cb74041b12f" xlink:href="rnac-20251231.xsd#rnac_LesseeOperatingLeaseNumberOfOptionsToExtend"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_rnac_LesseeOperatingLeaseNumberOfOptionsToExtend_8d2c7153-7ff8-4e3f-9c41-2cb74041b12f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_8c24182a-b49b-4581-9b13-192c6a0a5ac1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_8c24182a-b49b-4581-9b13-192c6a0a5ac1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OperatingLeaseCostPercentage_42e271a9-d640-4dda-98c1-e07ed0d60dc1" xlink:href="rnac-20251231.xsd#rnac_OperatingLeaseCostPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_rnac_OperatingLeaseCostPercentage_42e271a9-d640-4dda-98c1-e07ed0d60dc1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LesseeOperatingLeaseMonthlyBaseRent_bbb42ec2-39f5-4647-b27f-2afc8a5d1b52" xlink:href="rnac-20251231.xsd#rnac_LesseeOperatingLeaseMonthlyBaseRent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_rnac_LesseeOperatingLeaseMonthlyBaseRent_bbb42ec2-39f5-4647-b27f-2afc8a5d1b52" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LesseeOperatingLeaseBaseRentAnnualUpwardAdjustmentPercentageAfterYearOne_3a07e371-0199-427c-ba4b-6fa55c1b673b" xlink:href="rnac-20251231.xsd#rnac_LesseeOperatingLeaseBaseRentAnnualUpwardAdjustmentPercentageAfterYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_rnac_LesseeOperatingLeaseBaseRentAnnualUpwardAdjustmentPercentageAfterYearOne_3a07e371-0199-427c-ba4b-6fa55c1b673b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LesseeOperatingLeaseFirstMonthRentPaidOnExecutionOfLeaseAgreement_cd91737d-bec3-4a32-b46b-83489e26f08b" xlink:href="rnac-20251231.xsd#rnac_LesseeOperatingLeaseFirstMonthRentPaidOnExecutionOfLeaseAgreement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_rnac_LesseeOperatingLeaseFirstMonthRentPaidOnExecutionOfLeaseAgreement_cd91737d-bec3-4a32-b46b-83489e26f08b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_b51e9eb3-3f6a-4bfe-b82d-206b78ba50a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_us-gaap_OperatingLeaseLiability_b51e9eb3-3f6a-4bfe-b82d-206b78ba50a9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_cf241770-bf37-41f1-933a-274037d89ec2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_cf241770-bf37-41f1-933a-274037d89ec2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OperatingLeaseTenantImprovementAllowanceRecognizedAsReductionInRightOfUseAssetAndLeaseLiability_fd837856-8e9b-4af0-adef-fe05b2c1505b" xlink:href="rnac-20251231.xsd#rnac_OperatingLeaseTenantImprovementAllowanceRecognizedAsReductionInRightOfUseAssetAndLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_rnac_OperatingLeaseTenantImprovementAllowanceRecognizedAsReductionInRightOfUseAssetAndLeaseLiability_fd837856-8e9b-4af0-adef-fe05b2c1505b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_82934055-d76f-40b2-a433-a115422b588b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_82934055-d76f-40b2-a433-a115422b588b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LesseeOperatingLeaseAdditionalAnnualBaseRentYearOne_a0fa1425-1070-4e2c-ad42-5556115977a7" xlink:href="rnac-20251231.xsd#rnac_LesseeOperatingLeaseAdditionalAnnualBaseRentYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_rnac_LesseeOperatingLeaseAdditionalAnnualBaseRentYearOne_a0fa1425-1070-4e2c-ad42-5556115977a7" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_f234828b-ccda-4e6f-8531-0a0665342542" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_bbcdbf7e-e671-4665-aeda-d300b88e21cc" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_f234828b-ccda-4e6f-8531-0a0665342542" xlink:type="arc" order="13"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/Leases65GroveStreetLeaseNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#Leases65GroveStreetLeaseNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/Leases65GroveStreetLeaseNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeasedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_cd59a108-9802-4f5a-bdac-82acc56f4c3d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfOperatingLeasedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_cd59a108-9802-4f5a-bdac-82acc56f4c3d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementAxis_f2b7596b-c007-4d57-9956-935073c960be" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_cd59a108-9802-4f5a-bdac-82acc56f4c3d" xlink:to="loc_rnac_LeaseArrangementAxis_f2b7596b-c007-4d57-9956-935073c960be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementDomain_f2b7596b-c007-4d57-9956-935073c960be_default" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_rnac_LeaseArrangementAxis_f2b7596b-c007-4d57-9956-935073c960be" xlink:to="loc_rnac_LeaseArrangementDomain_f2b7596b-c007-4d57-9956-935073c960be_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementDomain_ee7996cc-ebac-4316-8fc3-b92f33eacc77" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_rnac_LeaseArrangementAxis_f2b7596b-c007-4d57-9956-935073c960be" xlink:to="loc_rnac_LeaseArrangementDomain_ee7996cc-ebac-4316-8fc3-b92f33eacc77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_A65GroveStreetWatertownMAMember_10efa6a9-bc13-4d77-8d2d-cc9801b67dfe" xlink:href="rnac-20251231.xsd#rnac_A65GroveStreetWatertownMAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_LeaseArrangementDomain_ee7996cc-ebac-4316-8fc3-b92f33eacc77" xlink:to="loc_rnac_A65GroveStreetWatertownMAMember_10efa6a9-bc13-4d77-8d2d-cc9801b67dfe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_5c39cb04-aa26-4df7-ae12-aa17e4d3ce94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_cd59a108-9802-4f5a-bdac-82acc56f4c3d" xlink:to="loc_us-gaap_CreditFacilityAxis_5c39cb04-aa26-4df7-ae12-aa17e4d3ce94" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_5c39cb04-aa26-4df7-ae12-aa17e4d3ce94_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_5c39cb04-aa26-4df7-ae12-aa17e4d3ce94" xlink:to="loc_us-gaap_CreditFacilityDomain_5c39cb04-aa26-4df7-ae12-aa17e4d3ce94_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_7d2ddc7f-3dd4-4755-be9e-83336f52f132" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_5c39cb04-aa26-4df7-ae12-aa17e4d3ce94" xlink:to="loc_us-gaap_CreditFacilityDomain_7d2ddc7f-3dd4-4755-be9e-83336f52f132" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_cf17cf59-8d25-4f91-9a91-c917b95923ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_7d2ddc7f-3dd4-4755-be9e-83336f52f132" xlink:to="loc_us-gaap_LetterOfCreditMember_cf17cf59-8d25-4f91-9a91-c917b95923ae" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_a46c17de-30b7-4c75-a79b-8832ef0d8171" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AreaOfRealEstateProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_a46c17de-30b7-4c75-a79b-8832ef0d8171" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_1737be6d-a063-4a79-ad93-0869212a3fe5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_LeaseCost_1737be6d-a063-4a79-ad93-0869212a3fe5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_7c4628f9-83c8-41c7-879b-e259c6a8e3f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_7c4628f9-83c8-41c7-879b-e259c6a8e3f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_e0b8dcb6-d53f-452a-8b39-847ad4723c10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_e0b8dcb6-d53f-452a-8b39-847ad4723c10" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCash_1db125bf-28c5-4cb1-8dc1-10e56a744550" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_RestrictedCash_1db125bf-28c5-4cb1-8dc1-10e56a744550" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OperatingLeaseBaseRent_79cf0543-6cdd-4b6c-ba51-2a2aef2bab21" xlink:href="rnac-20251231.xsd#rnac_OperatingLeaseBaseRent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_rnac_OperatingLeaseBaseRent_79cf0543-6cdd-4b6c-ba51-2a2aef2bab21" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_6fe17ed6-42ee-4609-81ba-b05762301565" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_6fe17ed6-42ee-4609-81ba-b05762301565" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_a8f15b87-f01f-45e2-bac9-91a174628d3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_OperatingLeaseLiability_a8f15b87-f01f-45e2-bac9-91a174628d3b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashNoncurrent_d6cdbd1d-2674-49a7-93ee-e4dd56a16365" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_RestrictedCashNoncurrent_d6cdbd1d-2674-49a7-93ee-e4dd56a16365" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_UpfrontRentalPayment_27ab3802-c057-4578-a3b2-943379d346a5" xlink:href="rnac-20251231.xsd#rnac_UpfrontRentalPayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_rnac_UpfrontRentalPayment_27ab3802-c057-4578-a3b2-943379d346a5" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetImpairmentCharges_3652937e-443d-4f74-9f19-4e452ab3a4d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_AssetImpairmentCharges_3652937e-443d-4f74-9f19-4e452ab3a4d3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseImpairmentLoss_58361070-4657-4525-be98-3f05326239fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_OperatingLeaseImpairmentLoss_58361070-4657-4525-be98-3f05326239fa" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_039daa8a-62fd-4220-9c8f-675209ac1008" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_b73072f1-b744-4d88-bbc4-6dba5e46af60" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_039daa8a-62fd-4220-9c8f-675209ac1008" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentLongLivedAssetHeldForUseStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_86f5c173-10ca-418f-99d8-b3fad9259fc9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentLongLivedAssetHeldForUseStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_039daa8a-62fd-4220-9c8f-675209ac1008" xlink:to="loc_us-gaap_ImpairmentLongLivedAssetHeldForUseStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_86f5c173-10ca-418f-99d8-b3fad9259fc9" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/Leases704QuinceOrchardRoadLeasesNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#Leases704QuinceOrchardRoadLeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/Leases704QuinceOrchardRoadLeasesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasedAssetsLineItems_d331e01b-4780-4963-9fa6-1ef75065767c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeasedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_dcfdb75d-9812-40bd-8e03-00d220b798bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfOperatingLeasedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_d331e01b-4780-4963-9fa6-1ef75065767c" xlink:to="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_dcfdb75d-9812-40bd-8e03-00d220b798bf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementAxis_7d5ba198-3688-4357-aeed-b6f77808fd32" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_dcfdb75d-9812-40bd-8e03-00d220b798bf" xlink:to="loc_rnac_LeaseArrangementAxis_7d5ba198-3688-4357-aeed-b6f77808fd32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementDomain_7d5ba198-3688-4357-aeed-b6f77808fd32_default" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_rnac_LeaseArrangementAxis_7d5ba198-3688-4357-aeed-b6f77808fd32" xlink:to="loc_rnac_LeaseArrangementDomain_7d5ba198-3688-4357-aeed-b6f77808fd32_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LeaseArrangementDomain_7a326fe7-1bc2-4628-ab3a-9807dc7e2e38" xlink:href="rnac-20251231.xsd#rnac_LeaseArrangementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_rnac_LeaseArrangementAxis_7d5ba198-3688-4357-aeed-b6f77808fd32" xlink:to="loc_rnac_LeaseArrangementDomain_7a326fe7-1bc2-4628-ab3a-9807dc7e2e38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_A704QuinceOrchardRoadLeasesMember_485fb6d0-d7ea-4e9f-89b9-b885e4b9375b" xlink:href="rnac-20251231.xsd#rnac_A704QuinceOrchardRoadLeasesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_LeaseArrangementDomain_7a326fe7-1bc2-4628-ab3a-9807dc7e2e38" xlink:to="loc_rnac_A704QuinceOrchardRoadLeasesMember_485fb6d0-d7ea-4e9f-89b9-b885e4b9375b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_NumberOfAcquiredOperatingLeases_87348fcf-26c5-4cda-9270-4fa094d6e9dc" xlink:href="rnac-20251231.xsd#rnac_NumberOfAcquiredOperatingLeases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_d331e01b-4780-4963-9fa6-1ef75065767c" xlink:to="loc_rnac_NumberOfAcquiredOperatingLeases_87348fcf-26c5-4cda-9270-4fa094d6e9dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_00cd3158-d5b6-443b-a2ce-c01354f10658" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_d331e01b-4780-4963-9fa6-1ef75065767c" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_00cd3158-d5b6-443b-a2ce-c01354f10658" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_48c3a3da-c388-4f78-93d9-493720edb404" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_d331e01b-4780-4963-9fa6-1ef75065767c" xlink:to="loc_us-gaap_LeaseCost_48c3a3da-c388-4f78-93d9-493720edb404" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/ConvertiblePreferredStockSeriesBPreferredStockNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#ConvertiblePreferredStockSeriesBPreferredStockNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/ConvertiblePreferredStockSeriesBPreferredStockNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019c2e84-6ce3-71c8-83f8-1a3e74035851" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019c2e84-6ce3-71c8-83f8-1a3e74035851" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019c2e84-6ce3-73d3-bb99-ae4d7243e893" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019c2e84-6ce3-71c8-83f8-1a3e74035851" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019c2e84-6ce3-73d3-bb99-ae4d7243e893" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019c2e84-6ce3-73d3-bb99-ae4d7243e893_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019c2e84-6ce3-73d3-bb99-ae4d7243e893" xlink:to="loc_us-gaap_ClassOfStockDomain_019c2e84-6ce3-73d3-bb99-ae4d7243e893_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019c2e84-6ce3-7248-a7b3-9bd32523d3e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019c2e84-6ce3-73d3-bb99-ae4d7243e893" xlink:to="loc_us-gaap_ClassOfStockDomain_019c2e84-6ce3-7248-a7b3-9bd32523d3e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_019c2e84-6ce3-76e4-9de5-149d34040b12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019c2e84-6ce3-7248-a7b3-9bd32523d3e1" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_019c2e84-6ce3-76e4-9de5-149d34040b12" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019c2e84-6ce3-70b5-86d5-75f76f58ced2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019c2e84-6ce3-7248-a7b3-9bd32523d3e1" xlink:to="loc_us-gaap_CommonStockMember_019c2e84-6ce3-70b5-86d5-75f76f58ced2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019c2e84-6ce3-7400-865d-25060c66fb16" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019c2e84-6ce3-71c8-83f8-1a3e74035851" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019c2e84-6ce3-7400-865d-25060c66fb16" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c2e84-6ce3-7400-865d-25060c66fb16_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019c2e84-6ce3-7400-865d-25060c66fb16" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c2e84-6ce3-7400-865d-25060c66fb16_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c2e84-6ce3-7fa3-b3c4-22eb50a57ead" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019c2e84-6ce3-7400-865d-25060c66fb16" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c2e84-6ce3-7fa3-b3c4-22eb50a57ead" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_July2024PurchaseAgreementMember_019c2e84-6ce3-7bb6-927a-26357b166bc1" xlink:href="rnac-20251231.xsd#rnac_July2024PurchaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c2e84-6ce3-7fa3-b3c4-22eb50a57ead" xlink:to="loc_rnac_July2024PurchaseAgreementMember_019c2e84-6ce3-7bb6-927a-26357b166bc1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SecuritiesPurchaseAgreement2024Member_019c2e84-6ce3-7235-a42f-2bf37973338c" xlink:href="rnac-20251231.xsd#rnac_SecuritiesPurchaseAgreement2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c2e84-6ce3-7fa3-b3c4-22eb50a57ead" xlink:to="loc_rnac_SecuritiesPurchaseAgreement2024Member_019c2e84-6ce3-7235-a42f-2bf37973338c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019c2e84-6ce3-7c14-bdfa-86fb91ed784e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019c2e84-6ce3-71c8-83f8-1a3e74035851" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019c2e84-6ce3-7c14-bdfa-86fb91ed784e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019c2e84-6ce3-7c14-bdfa-86fb91ed784e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019c2e84-6ce3-7c14-bdfa-86fb91ed784e" xlink:to="loc_us-gaap_EquityComponentDomain_019c2e84-6ce3-7c14-bdfa-86fb91ed784e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019c2e84-6ce3-7cdb-aa87-5c3b67210e2f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019c2e84-6ce3-7c14-bdfa-86fb91ed784e" xlink:to="loc_us-gaap_EquityComponentDomain_019c2e84-6ce3-7cdb-aa87-5c3b67210e2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019c2e84-6ce3-7eb8-ae86-b7d2bd6d8ecc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c2e84-6ce3-7cdb-aa87-5c3b67210e2f" xlink:to="loc_us-gaap_CommonStockMember_019c2e84-6ce3-7eb8-ae86-b7d2bd6d8ecc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019c2e84-6ce3-7e91-bbd7-c7dfc410bb1d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019c2e84-6ce3-71c8-83f8-1a3e74035851" xlink:to="loc_srt_CounterpartyNameAxis_019c2e84-6ce3-7e91-bbd7-c7dfc410bb1d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c2e84-6ce3-7e91-bbd7-c7dfc410bb1d_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019c2e84-6ce3-7e91-bbd7-c7dfc410bb1d" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c2e84-6ce3-7e91-bbd7-c7dfc410bb1d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c2e84-6ce3-73b3-880a-a4140cd5876e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019c2e84-6ce3-7e91-bbd7-c7dfc410bb1d" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c2e84-6ce3-73b3-880a-a4140cd5876e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_Dr.TimothyA.SpringerMember_019c2e84-6ce3-7b93-93d8-4971394f1f7a" xlink:href="rnac-20251231.xsd#rnac_Dr.TimothyA.SpringerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c2e84-6ce3-73b3-880a-a4140cd5876e" xlink:to="loc_rnac_Dr.TimothyA.SpringerMember_019c2e84-6ce3-7b93-93d8-4971394f1f7a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TASPartnersLLCMember_019c2e84-6ce3-7e73-aa01-400d36b476a4" xlink:href="rnac-20251231.xsd#rnac_TASPartnersLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c2e84-6ce3-73b3-880a-a4140cd5876e" xlink:to="loc_rnac_TASPartnersLLCMember_019c2e84-6ce3-7e73-aa01-400d36b476a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_Dr.ChafenLuMember_019c2e84-6ce3-744f-b7b8-0014a12e0c0b" xlink:href="rnac-20251231.xsd#rnac_Dr.ChafenLuMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c2e84-6ce3-73b3-880a-a4140cd5876e" xlink:to="loc_rnac_Dr.ChafenLuMember_019c2e84-6ce3-744f-b7b8-0014a12e0c0b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019c2e84-6ce3-759a-8a68-73dca50d74a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019c2e84-6ce3-71c8-83f8-1a3e74035851" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019c2e84-6ce3-759a-8a68-73dca50d74a2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019c2e84-6ce3-759a-8a68-73dca50d74a2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019c2e84-6ce3-759a-8a68-73dca50d74a2" xlink:to="loc_us-gaap_RelatedPartyDomain_019c2e84-6ce3-759a-8a68-73dca50d74a2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019c2e84-6ce3-7d39-80d8-37f6d966a01a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019c2e84-6ce3-759a-8a68-73dca50d74a2" xlink:to="loc_us-gaap_RelatedPartyDomain_019c2e84-6ce3-7d39-80d8-37f6d966a01a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019c2e84-6ce3-7a8d-a218-979f96f88dd9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019c2e84-6ce3-7d39-80d8-37f6d966a01a" xlink:to="loc_us-gaap_RelatedPartyMember_019c2e84-6ce3-7a8d-a218-979f96f88dd9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c2e85-b627-7358-91b0-599be22e3745" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019c2e84-6ce3-71c8-83f8-1a3e74035851" xlink:to="loc_srt_RangeAxis_019c2e85-b627-7358-91b0-599be22e3745" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c2e85-b627-7358-91b0-599be22e3745_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c2e85-b627-7358-91b0-599be22e3745" xlink:to="loc_srt_RangeMember_019c2e85-b627-7358-91b0-599be22e3745_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c2e85-b627-7a82-8e58-8be16c30ba00" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c2e85-b627-7358-91b0-599be22e3745" xlink:to="loc_srt_RangeMember_019c2e85-b627-7a82-8e58-8be16c30ba00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c2e85-b627-7e92-b655-3db394724e76" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c2e85-b627-7a82-8e58-8be16c30ba00" xlink:to="loc_srt_MinimumMember_019c2e85-b627-7e92-b655-3db394724e76" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c2e85-b627-7643-8e2e-18f1713b46d0" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c2e85-b627-7a82-8e58-8be16c30ba00" xlink:to="loc_srt_MaximumMember_019c2e85-b627-7643-8e2e-18f1713b46d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019c2e84-6ce3-7e47-8845-579d28716437" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019c2e84-6ce3-7e47-8845-579d28716437" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019c2e84-6ce3-7dba-8da0-7c748625df25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019c2e84-6ce3-7dba-8da0-7c748625df25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019c2e84-6ce3-7317-bc82-1d650332c292" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019c2e84-6ce3-7317-bc82-1d650332c292" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019c2e84-6ce3-739f-b5c1-d69d6e9ed417" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019c2e84-6ce3-739f-b5c1-d69d6e9ed417" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SaleOfStockGrossConsiderationReceivedOnTransaction_019c2e84-6ce3-7ab5-adc1-27aac5d98144" xlink:href="rnac-20251231.xsd#rnac_SaleOfStockGrossConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_rnac_SaleOfStockGrossConsiderationReceivedOnTransaction_019c2e84-6ce3-7ab5-adc1-27aac5d98144" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_019c2e84-6ce3-71c4-84c2-d1851bb0b546" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_019c2e84-6ce3-71c4-84c2-d1851bb0b546" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019c2e84-6ce3-7650-8426-ca7c8407211f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019c2e84-6ce3-7650-8426-ca7c8407211f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_019c2e84-6ce3-7724-a285-8c5614d7ff47" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_019c2e84-6ce3-7724-a285-8c5614d7ff47" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ConvertiblePreferredStockSharesThatDidNotAutomaticallyConvert_019c2e84-6ce3-790c-a54d-412c60ddc25d" xlink:href="rnac-20251231.xsd#rnac_ConvertiblePreferredStockSharesThatDidNotAutomaticallyConvert"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019c2e84-6ce3-7845-84f6-572ca6eb2299" xlink:to="loc_rnac_ConvertiblePreferredStockSharesThatDidNotAutomaticallyConvert_019c2e84-6ce3-790c-a54d-412c60ddc25d" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/ConvertiblePreferredStockConversionNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#ConvertiblePreferredStockConversionNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/ConvertiblePreferredStockConversionNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_67f3f7b5-91ee-43de-aa01-51ca05af8b33" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_b083e5f4-0f12-4e82-8e53-50d78b2355ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_67f3f7b5-91ee-43de-aa01-51ca05af8b33" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_b083e5f4-0f12-4e82-8e53-50d78b2355ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_517a44dc-e27b-4a01-bd31-7fd3a126920d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_b083e5f4-0f12-4e82-8e53-50d78b2355ac" xlink:to="loc_srt_RangeAxis_517a44dc-e27b-4a01-bd31-7fd3a126920d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_517a44dc-e27b-4a01-bd31-7fd3a126920d_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_517a44dc-e27b-4a01-bd31-7fd3a126920d" xlink:to="loc_srt_RangeMember_517a44dc-e27b-4a01-bd31-7fd3a126920d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b5622705-a6b7-4e65-a7b6-658126cd7a69" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_517a44dc-e27b-4a01-bd31-7fd3a126920d" xlink:to="loc_srt_RangeMember_b5622705-a6b7-4e65-a7b6-658126cd7a69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_c3c43cad-4603-4929-843d-a2691f586931" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b5622705-a6b7-4e65-a7b6-658126cd7a69" xlink:to="loc_srt_MinimumMember_c3c43cad-4603-4929-843d-a2691f586931" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_b1fea063-ab09-4b2c-ab9e-5a8002838317" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b5622705-a6b7-4e65-a7b6-658126cd7a69" xlink:to="loc_srt_MaximumMember_b1fea063-ab09-4b2c-ab9e-5a8002838317" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_c834e214-2984-4a8a-9d4c-2ee9fb75c0a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_b083e5f4-0f12-4e82-8e53-50d78b2355ac" xlink:to="loc_us-gaap_StatementClassOfStockAxis_c834e214-2984-4a8a-9d4c-2ee9fb75c0a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c834e214-2984-4a8a-9d4c-2ee9fb75c0a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c834e214-2984-4a8a-9d4c-2ee9fb75c0a1" xlink:to="loc_us-gaap_ClassOfStockDomain_c834e214-2984-4a8a-9d4c-2ee9fb75c0a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b80fbd3b-b662-4d3b-9792-027d8d7dfd42" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c834e214-2984-4a8a-9d4c-2ee9fb75c0a1" xlink:to="loc_us-gaap_ClassOfStockDomain_b80fbd3b-b662-4d3b-9792-027d8d7dfd42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_18a4c66d-4415-4289-a40d-996c4ebaef70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_b80fbd3b-b662-4d3b-9792-027d8d7dfd42" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_18a4c66d-4415-4289-a40d-996c4ebaef70" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockConvertibleConversionRatio_2c9a08a2-6a8c-49a1-98a3-a46f64252c15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockConvertibleConversionRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_67f3f7b5-91ee-43de-aa01-51ca05af8b33" xlink:to="loc_us-gaap_PreferredStockConvertibleConversionRatio_2c9a08a2-6a8c-49a1-98a3-a46f64252c15" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_PreferredStockConvertibleBeneficialOwnershipPercentage_4ebda461-4c8c-4e7f-a1db-6ff3a4ddc6cc" xlink:href="rnac-20251231.xsd#rnac_PreferredStockConvertibleBeneficialOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_67f3f7b5-91ee-43de-aa01-51ca05af8b33" xlink:to="loc_rnac_PreferredStockConvertibleBeneficialOwnershipPercentage_4ebda461-4c8c-4e7f-a1db-6ff3a4ddc6cc" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/ConvertiblePreferredStockSeriesAPreferredStockNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#ConvertiblePreferredStockSeriesAPreferredStockNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/ConvertiblePreferredStockSeriesAPreferredStockNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_61f0738b-c7cc-4396-81bd-0bce46c005cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_61f0738b-c7cc-4396-81bd-0bce46c005cc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_24554461-a8ab-4427-9dea-6d3640c308a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_61f0738b-c7cc-4396-81bd-0bce46c005cc" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_24554461-a8ab-4427-9dea-6d3640c308a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_24554461-a8ab-4427-9dea-6d3640c308a0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_24554461-a8ab-4427-9dea-6d3640c308a0" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_24554461-a8ab-4427-9dea-6d3640c308a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_f3a91879-ad2f-41c2-b6b7-f0fbe16e6555" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_24554461-a8ab-4427-9dea-6d3640c308a0" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_f3a91879-ad2f-41c2-b6b7-f0fbe16e6555" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SecuritiesPurchaseAgreement2023Member_cfa493da-beeb-44cc-bcbf-c31e2fe6a314" xlink:href="rnac-20251231.xsd#rnac_SecuritiesPurchaseAgreement2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_f3a91879-ad2f-41c2-b6b7-f0fbe16e6555" xlink:to="loc_rnac_SecuritiesPurchaseAgreement2023Member_cfa493da-beeb-44cc-bcbf-c31e2fe6a314" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_PrivatePlacement2023Member_5e0db5a8-79e2-40c2-913d-8225cdd5b3be" xlink:href="rnac-20251231.xsd#rnac_PrivatePlacement2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_f3a91879-ad2f-41c2-b6b7-f0fbe16e6555" xlink:to="loc_rnac_PrivatePlacement2023Member_5e0db5a8-79e2-40c2-913d-8225cdd5b3be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_PrivatePlacement2023TrancheOneMember_b804d81c-f0e0-4e5d-af35-07e01c3bd6a1" xlink:href="rnac-20251231.xsd#rnac_PrivatePlacement2023TrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_PrivatePlacement2023Member_5e0db5a8-79e2-40c2-913d-8225cdd5b3be" xlink:to="loc_rnac_PrivatePlacement2023TrancheOneMember_b804d81c-f0e0-4e5d-af35-07e01c3bd6a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_PrivatePlacement2023TrancheTwoMember_687a7945-1f0e-41d5-a7b4-e88c343016ab" xlink:href="rnac-20251231.xsd#rnac_PrivatePlacement2023TrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_PrivatePlacement2023Member_5e0db5a8-79e2-40c2-913d-8225cdd5b3be" xlink:to="loc_rnac_PrivatePlacement2023TrancheTwoMember_687a7945-1f0e-41d5-a7b4-e88c343016ab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_PrivatePlacement2023TrancheThreeMember_7ae54a61-4bf4-4236-975f-93afde022cca" xlink:href="rnac-20251231.xsd#rnac_PrivatePlacement2023TrancheThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_PrivatePlacement2023Member_5e0db5a8-79e2-40c2-913d-8225cdd5b3be" xlink:to="loc_rnac_PrivatePlacement2023TrancheThreeMember_7ae54a61-4bf4-4236-975f-93afde022cca" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_93f6ccbd-75a1-410f-a8e9-32866f3945c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_61f0738b-c7cc-4396-81bd-0bce46c005cc" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_93f6ccbd-75a1-410f-a8e9-32866f3945c7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_93f6ccbd-75a1-410f-a8e9-32866f3945c7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_93f6ccbd-75a1-410f-a8e9-32866f3945c7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_93f6ccbd-75a1-410f-a8e9-32866f3945c7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e28a03a7-620b-4ed6-bf8e-bd8aac6ddecb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_93f6ccbd-75a1-410f-a8e9-32866f3945c7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e28a03a7-620b-4ed6-bf8e-bd8aac6ddecb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OldCartesianMember_e22a22d0-8c35-486d-852b-c789c1d37d97" xlink:href="rnac-20251231.xsd#rnac_OldCartesianMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e28a03a7-620b-4ed6-bf8e-bd8aac6ddecb" xlink:to="loc_rnac_OldCartesianMember_e22a22d0-8c35-486d-852b-c789c1d37d97" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_f27f1aee-8418-4eb5-a490-787152b4c87f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityInterestIssuedOrIssuableByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_61f0738b-c7cc-4396-81bd-0bce46c005cc" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_f27f1aee-8418-4eb5-a490-787152b4c87f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_f27f1aee-8418-4eb5-a490-787152b4c87f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_f27f1aee-8418-4eb5-a490-787152b4c87f" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_f27f1aee-8418-4eb5-a490-787152b4c87f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_2dcea133-d15c-4a12-aa75-e26b8cb5ac01" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityInterestIssuedOrIssuableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableByTypeAxis_f27f1aee-8418-4eb5-a490-787152b4c87f" xlink:to="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_2dcea133-d15c-4a12-aa75-e26b8cb5ac01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_a8d909c9-39f9-438d-86bc-ad61a334bb74" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityInterestIssuedOrIssuableTypeDomain_2dcea133-d15c-4a12-aa75-e26b8cb5ac01" xlink:to="loc_us-gaap_PreferredStockMember_a8d909c9-39f9-438d-86bc-ad61a334bb74" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_22074af6-f852-42bc-9b20-597e08e20760" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_61f0738b-c7cc-4396-81bd-0bce46c005cc" xlink:to="loc_us-gaap_StatementClassOfStockAxis_22074af6-f852-42bc-9b20-597e08e20760" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_22074af6-f852-42bc-9b20-597e08e20760_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_22074af6-f852-42bc-9b20-597e08e20760" xlink:to="loc_us-gaap_ClassOfStockDomain_22074af6-f852-42bc-9b20-597e08e20760_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_9a9ae7dd-d271-40da-a009-a58b5dbe70dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_22074af6-f852-42bc-9b20-597e08e20760" xlink:to="loc_us-gaap_ClassOfStockDomain_9a9ae7dd-d271-40da-a009-a58b5dbe70dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_ad823bd9-80b1-46f1-abe6-96f60c7e19c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9a9ae7dd-d271-40da-a009-a58b5dbe70dd" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_ad823bd9-80b1-46f1-abe6-96f60c7e19c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_1cecf255-5b2f-4502-80c4-d05cc0eb4eb3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_61f0738b-c7cc-4396-81bd-0bce46c005cc" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_1cecf255-5b2f-4502-80c4-d05cc0eb4eb3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_1cecf255-5b2f-4502-80c4-d05cc0eb4eb3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_1cecf255-5b2f-4502-80c4-d05cc0eb4eb3" xlink:to="loc_us-gaap_EquityComponentDomain_1cecf255-5b2f-4502-80c4-d05cc0eb4eb3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_b3357df4-f7cf-4d18-b469-0797b1d1dac5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_1cecf255-5b2f-4502-80c4-d05cc0eb4eb3" xlink:to="loc_us-gaap_EquityComponentDomain_b3357df4-f7cf-4d18-b469-0797b1d1dac5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_71eaeefd-b706-4de0-8386-4b1ec44d854e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_b3357df4-f7cf-4d18-b469-0797b1d1dac5" xlink:to="loc_us-gaap_CommonStockMember_71eaeefd-b706-4de0-8386-4b1ec44d854e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesAuthorized_162dce40-5c5e-4ed8-b011-04b3e1e169db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_us-gaap_TemporaryEquitySharesAuthorized_162dce40-5c5e-4ed8-b011-04b3e1e169db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_bcf95551-b99b-4e1d-91ec-12c994d97946" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_bcf95551-b99b-4e1d-91ec-12c994d97946" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_f486df47-02ce-42dc-ac8d-8c19775b784b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_f486df47-02ce-42dc-ac8d-8c19775b784b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SaleOfStockSettlementTerm_70084e69-388d-4854-9a57-f603489dd33e" xlink:href="rnac-20251231.xsd#rnac_SaleOfStockSettlementTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_rnac_SaleOfStockSettlementTerm_70084e69-388d-4854-9a57-f603489dd33e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SettlementOfDerivativeLiabilityShares_25de11b0-e4ca-44b9-935d-a6c3a97d5fd0" xlink:href="rnac-20251231.xsd#rnac_SettlementOfDerivativeLiabilityShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_rnac_SettlementOfDerivativeLiabilityShares_25de11b0-e4ca-44b9-935d-a6c3a97d5fd0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_9f4dd90e-80d0-44ec-8bf4-c4fcf9e48498" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_9f4dd90e-80d0-44ec-8bf4-c4fcf9e48498" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_NumberOfBusinessDays_967f2a74-285f-4d60-b417-5311d9ac0b90" xlink:href="rnac-20251231.xsd#rnac_NumberOfBusinessDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_rnac_NumberOfBusinessDays_967f2a74-285f-4d60-b417-5311d9ac0b90" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_ed11db66-b4ed-42b3-8acc-70384060c3c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_ed11db66-b4ed-42b3-8acc-70384060c3c5" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ConvertiblePreferredStockSharesThatDidNotAutomaticallyConvert_6f1531f1-7560-4268-b4da-0cb2f0034817" xlink:href="rnac-20251231.xsd#rnac_ConvertiblePreferredStockSharesThatDidNotAutomaticallyConvert"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_32325b90-767e-490e-b275-1c239eb1f77c" xlink:to="loc_rnac_ConvertiblePreferredStockSharesThatDidNotAutomaticallyConvert_6f1531f1-7560-4268-b4da-0cb2f0034817" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/ConvertiblePreferredStockLiquidationNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#ConvertiblePreferredStockLiquidationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/ConvertiblePreferredStockLiquidationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_d9d65e74-61da-4d6b-ae85-4a324de2e06e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_9d29c4ff-5146-46fb-a163-92b0b1869666" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_d9d65e74-61da-4d6b-ae85-4a324de2e06e" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_9d29c4ff-5146-46fb-a163-92b0b1869666" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_70df491c-481e-42bc-bf31-50adea81847d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_9d29c4ff-5146-46fb-a163-92b0b1869666" xlink:to="loc_us-gaap_StatementClassOfStockAxis_70df491c-481e-42bc-bf31-50adea81847d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_70df491c-481e-42bc-bf31-50adea81847d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_70df491c-481e-42bc-bf31-50adea81847d" xlink:to="loc_us-gaap_ClassOfStockDomain_70df491c-481e-42bc-bf31-50adea81847d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_6c00954f-badc-4636-8018-149c426164e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_70df491c-481e-42bc-bf31-50adea81847d" xlink:to="loc_us-gaap_ClassOfStockDomain_6c00954f-badc-4636-8018-149c426164e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_6367585f-99d1-4d7c-b9e8-bba533c8a9c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_6c00954f-badc-4636-8018-149c426164e6" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_6367585f-99d1-4d7c-b9e8-bba533c8a9c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_c4e22c2c-ac03-48b1-b350-1ef517977b8c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_6c00954f-badc-4636-8018-149c426164e6" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_c4e22c2c-ac03-48b1-b350-1ef517977b8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_0cf64579-8a8b-40ed-bc00-67c969b30cdb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d9d65e74-61da-4d6b-ae85-4a324de2e06e" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_0cf64579-8a8b-40ed-bc00-67c969b30cdb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_2ed424be-e383-49fa-bd6a-abf2b6526081" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d9d65e74-61da-4d6b-ae85-4a324de2e06e" xlink:to="loc_us-gaap_PreferredStockSharesIssued_2ed424be-e383-49fa-bd6a-abf2b6526081" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockConvertibleSharesIssuable_99a0bb66-0b99-490c-b2b8-ba5f34a8504a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockConvertibleSharesIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d9d65e74-61da-4d6b-ae85-4a324de2e06e" xlink:to="loc_us-gaap_PreferredStockConvertibleSharesIssuable_99a0bb66-0b99-490c-b2b8-ba5f34a8504a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/EquityAttheMarketSalesAgreementNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#EquityAttheMarketSalesAgreementNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/EquityAttheMarketSalesAgreementNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_9f4c2f41-1539-43e5-9c95-84dfa563b086" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_860f1e05-8cd7-47b3-9795-90b66af86365" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_9f4c2f41-1539-43e5-9c95-84dfa563b086" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_860f1e05-8cd7-47b3-9795-90b66af86365" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_bc72ddea-ac84-4d95-aedf-242c53016163" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_860f1e05-8cd7-47b3-9795-90b66af86365" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_bc72ddea-ac84-4d95-aedf-242c53016163" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_bc72ddea-ac84-4d95-aedf-242c53016163_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_bc72ddea-ac84-4d95-aedf-242c53016163" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_bc72ddea-ac84-4d95-aedf-242c53016163_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5e683342-99da-4564-bc71-096e3e7a1383" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_bc72ddea-ac84-4d95-aedf-242c53016163" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5e683342-99da-4564-bc71-096e3e7a1383" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_AtTheMarketOfferingMember_64e49e72-f1ce-489a-b8aa-500843608fab" xlink:href="rnac-20251231.xsd#rnac_AtTheMarketOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5e683342-99da-4564-bc71-096e3e7a1383" xlink:to="loc_rnac_AtTheMarketOfferingMember_64e49e72-f1ce-489a-b8aa-500843608fab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SaleOfStockAggregateMaximumGrossSalesProceeds_bdbc9f15-0edd-49a5-bdbe-c4807193b32e" xlink:href="rnac-20251231.xsd#rnac_SaleOfStockAggregateMaximumGrossSalesProceeds"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_9f4c2f41-1539-43e5-9c95-84dfa563b086" xlink:to="loc_rnac_SaleOfStockAggregateMaximumGrossSalesProceeds_bdbc9f15-0edd-49a5-bdbe-c4807193b32e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/Equity2024PrivatePlacementNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#Equity2024PrivatePlacementNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/Equity2024PrivatePlacementNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019bc5d0-d409-7bdf-9df0-75831511523b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019bc5d0-d409-7a7a-8da2-7f23e240535e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019bc5d0-d409-7bdf-9df0-75831511523b" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019bc5d0-d409-7a7a-8da2-7f23e240535e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019bc5d0-d409-75a8-b448-982dcedccfb2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019bc5d0-d409-7a7a-8da2-7f23e240535e" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019bc5d0-d409-75a8-b448-982dcedccfb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019bc5d0-d409-75a8-b448-982dcedccfb2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019bc5d0-d409-75a8-b448-982dcedccfb2" xlink:to="loc_us-gaap_ClassOfStockDomain_019bc5d0-d409-75a8-b448-982dcedccfb2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019bc5d0-d409-7adb-9206-5f5a39a5696d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019bc5d0-d409-75a8-b448-982dcedccfb2" xlink:to="loc_us-gaap_ClassOfStockDomain_019bc5d0-d409-7adb-9206-5f5a39a5696d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019bc5d0-d409-77c5-b7f7-f1acd12dda68" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019bc5d0-d409-7adb-9206-5f5a39a5696d" xlink:to="loc_us-gaap_CommonStockMember_019bc5d0-d409-77c5-b7f7-f1acd12dda68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_019bc5d0-d409-7fa2-871b-a20bfb175cb0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019bc5d0-d409-7adb-9206-5f5a39a5696d" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_019bc5d0-d409-7fa2-871b-a20bfb175cb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019bc5d0-d409-791a-bfac-0aa8644bcf4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019bc5d0-d409-7a7a-8da2-7f23e240535e" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019bc5d0-d409-791a-bfac-0aa8644bcf4f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc5d0-d409-791a-bfac-0aa8644bcf4f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019bc5d0-d409-791a-bfac-0aa8644bcf4f" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc5d0-d409-791a-bfac-0aa8644bcf4f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc5d0-d409-753d-ba81-f05512c0150a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019bc5d0-d409-791a-bfac-0aa8644bcf4f" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc5d0-d409-753d-ba81-f05512c0150a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_July2024PurchaseAgreementMember_019bc5d0-d409-779d-93ed-9d8f2d0796e8" xlink:href="rnac-20251231.xsd#rnac_July2024PurchaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc5d0-d409-753d-ba81-f05512c0150a" xlink:to="loc_rnac_July2024PurchaseAgreementMember_019bc5d0-d409-779d-93ed-9d8f2d0796e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019bc5d0-d409-7276-b17d-1ac88fc6e6a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019bc5d0-d409-7bdf-9df0-75831511523b" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019bc5d0-d409-7276-b17d-1ac88fc6e6a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019bc5d0-d409-7cf8-9775-380e3cdba454" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019bc5d0-d409-7bdf-9df0-75831511523b" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019bc5d0-d409-7cf8-9775-380e3cdba454" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019bc5d0-d409-75fa-9cb7-1652bd56a8d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019bc5d0-d409-7bdf-9df0-75831511523b" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019bc5d0-d409-75fa-9cb7-1652bd56a8d9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SaleOfStockGrossConsiderationReceivedOnTransaction_019bc5d0-d409-7076-a7a7-c3781ffcf668" xlink:href="rnac-20251231.xsd#rnac_SaleOfStockGrossConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019bc5d0-d409-7bdf-9df0-75831511523b" xlink:to="loc_rnac_SaleOfStockGrossConsiderationReceivedOnTransaction_019bc5d0-d409-7076-a7a7-c3781ffcf668" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/EquityScheduleofWarrantActivityDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#EquityScheduleofWarrantActivityDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/EquityScheduleofWarrantActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightLineItems_0445f5a7-b90e-439f-af3c-ef08067aada6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightTable_757586b0-659d-448d-bc77-e4abdc7edc6c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_0445f5a7-b90e-439f-af3c-ef08067aada6" xlink:to="loc_us-gaap_ClassOfWarrantOrRightTable_757586b0-659d-448d-bc77-e4abdc7edc6c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_6aba7e85-f5bb-4368-9454-970f0c795582" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfWarrantOrRightTable_757586b0-659d-448d-bc77-e4abdc7edc6c" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_6aba7e85-f5bb-4368-9454-970f0c795582" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_6aba7e85-f5bb-4368-9454-970f0c795582_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6aba7e85-f5bb-4368-9454-970f0c795582" xlink:to="loc_us-gaap_EquityComponentDomain_6aba7e85-f5bb-4368-9454-970f0c795582_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_ebe05546-a65a-4622-8786-42958ceb6324" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6aba7e85-f5bb-4368-9454-970f0c795582" xlink:to="loc_us-gaap_EquityComponentDomain_ebe05546-a65a-4622-8786-42958ceb6324" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_7dd1379b-6049-44d5-8c86-3a3765ce63e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_ebe05546-a65a-4622-8786-42958ceb6324" xlink:to="loc_us-gaap_WarrantMember_7dd1379b-6049-44d5-8c86-3a3765ce63e9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_ccef6b1b-6d65-4685-8d6e-8c3a4f8c17d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfWarrantOrRightTable_757586b0-659d-448d-bc77-e4abdc7edc6c" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_ccef6b1b-6d65-4685-8d6e-8c3a4f8c17d9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_ccef6b1b-6d65-4685-8d6e-8c3a4f8c17d9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_ccef6b1b-6d65-4685-8d6e-8c3a4f8c17d9" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_ccef6b1b-6d65-4685-8d6e-8c3a4f8c17d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_43b629ed-744e-4a6a-a099-738420a3bcba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_ccef6b1b-6d65-4685-8d6e-8c3a4f8c17d9" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_43b629ed-744e-4a6a-a099-738420a3bcba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_0592fb9e-ae35-442b-a688-4968a65b5082" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_43b629ed-744e-4a6a-a099-738420a3bcba" xlink:to="loc_us-gaap_WarrantMember_0592fb9e-ae35-442b-a688-4968a65b5082" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ClassOfWarrantOrRightRollForward_e677bf48-8df8-436f-acc5-34e7dc5c2098" xlink:href="rnac-20251231.xsd#rnac_ClassOfWarrantOrRightRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_0445f5a7-b90e-439f-af3c-ef08067aada6" xlink:to="loc_rnac_ClassOfWarrantOrRightRollForward_e677bf48-8df8-436f-acc5-34e7dc5c2098" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_d37fa05c-d788-4322-8edb-9cddcc2fcf81" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ClassOfWarrantOrRightRollForward_e677bf48-8df8-436f-acc5-34e7dc5c2098" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_d37fa05c-d788-4322-8edb-9cddcc2fcf81" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_StockIssuedDuringPeriodSharesWarrantsExercised_04b56546-46cd-4e15-a0a6-19579dbf2d37" xlink:href="rnac-20251231.xsd#rnac_StockIssuedDuringPeriodSharesWarrantsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ClassOfWarrantOrRightRollForward_e677bf48-8df8-436f-acc5-34e7dc5c2098" xlink:to="loc_rnac_StockIssuedDuringPeriodSharesWarrantsExercised_04b56546-46cd-4e15-a0a6-19579dbf2d37" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_StockIssuedDuringPeriodSharesWarrantsCanceled_b5f52b06-5f2d-4836-9389-917c4ec5da44" xlink:href="rnac-20251231.xsd#rnac_StockIssuedDuringPeriodSharesWarrantsCanceled"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ClassOfWarrantOrRightRollForward_e677bf48-8df8-436f-acc5-34e7dc5c2098" xlink:to="loc_rnac_StockIssuedDuringPeriodSharesWarrantsCanceled_b5f52b06-5f2d-4836-9389-917c4ec5da44" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_b83b6d20-9dac-4061-ac80-f5e9cd83f554" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ClassOfWarrantOrRightWeightedAverageExercisePriceRollForward_ba3cbebd-5227-484b-bbc1-e05742058e5b" xlink:href="rnac-20251231.xsd#rnac_ClassOfWarrantOrRightWeightedAverageExercisePriceRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_0445f5a7-b90e-439f-af3c-ef08067aada6" xlink:to="loc_rnac_ClassOfWarrantOrRightWeightedAverageExercisePriceRollForward_ba3cbebd-5227-484b-bbc1-e05742058e5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_8478ea69-746e-4a36-9f12-2f0f2b9a7cac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ClassOfWarrantOrRightWeightedAverageExercisePriceRollForward_ba3cbebd-5227-484b-bbc1-e05742058e5b" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_8478ea69-746e-4a36-9f12-2f0f2b9a7cac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRightsExercises_c9d363ca-7ab1-4080-93bf-db13b56b43e6" xlink:href="rnac-20251231.xsd#rnac_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRightsExercises"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ClassOfWarrantOrRightWeightedAverageExercisePriceRollForward_ba3cbebd-5227-484b-bbc1-e05742058e5b" xlink:to="loc_rnac_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRightsExercises_c9d363ca-7ab1-4080-93bf-db13b56b43e6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_StockIssuedDuringWarrantedCanceledPricePerShare_ec8d1fb7-993b-4c64-8cfe-f903432d4b39" xlink:href="rnac-20251231.xsd#rnac_StockIssuedDuringWarrantedCanceledPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ClassOfWarrantOrRightWeightedAverageExercisePriceRollForward_ba3cbebd-5227-484b-bbc1-e05742058e5b" xlink:to="loc_rnac_StockIssuedDuringWarrantedCanceledPricePerShare_ec8d1fb7-993b-4c64-8cfe-f903432d4b39" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_981adaff-ae5c-4640-a651-cd0c33783d2d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/EquityWarrantsNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#EquityWarrantsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/EquityWarrantsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightLineItems_019c554c-e2d2-7b23-9e8f-86ef1863ad05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightTable_019c554c-e2d2-70fd-a8b0-4e81136a6b24" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_019c554c-e2d2-7b23-9e8f-86ef1863ad05" xlink:to="loc_us-gaap_ClassOfWarrantOrRightTable_019c554c-e2d2-70fd-a8b0-4e81136a6b24" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019c554c-e2d2-7927-9ce1-ee56f6666649" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfWarrantOrRightTable_019c554c-e2d2-70fd-a8b0-4e81136a6b24" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019c554c-e2d2-7927-9ce1-ee56f6666649" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c554c-e2d2-7927-9ce1-ee56f6666649_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019c554c-e2d2-7927-9ce1-ee56f6666649" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c554c-e2d2-7927-9ce1-ee56f6666649_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c554c-e2d2-7970-9745-5306418fe1ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019c554c-e2d2-7927-9ce1-ee56f6666649" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c554c-e2d2-7970-9745-5306418fe1ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_DecemberTwoThousandNineteenFinancingMember_019c554c-e2d2-7757-ac5b-1adbf122f120" xlink:href="rnac-20251231.xsd#rnac_DecemberTwoThousandNineteenFinancingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019c554c-e2d2-7970-9745-5306418fe1ab" xlink:to="loc_rnac_DecemberTwoThousandNineteenFinancingMember_019c554c-e2d2-7757-ac5b-1adbf122f120" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantDownRoundFeatureIncreaseDecreaseInEquityAmount1_019c554c-e2d2-7467-bbad-83a8826b322d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantDownRoundFeatureIncreaseDecreaseInEquityAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_019c554c-e2d2-7b23-9e8f-86ef1863ad05" xlink:to="loc_us-gaap_WarrantDownRoundFeatureIncreaseDecreaseInEquityAmount1_019c554c-e2d2-7467-bbad-83a8826b322d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/EquityScheduleofAuthorizedSharesofCommonStockforFutureIssuanceDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#EquityScheduleofAuthorizedSharesofCommonStockforFutureIssuanceDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/EquityScheduleofAuthorizedSharesofCommonStockforFutureIssuanceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_10d4d0e9-4e2d-4d45-ad11-26c200828d2c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_83ea005b-5e77-47a0-87ee-bb4bcbb083a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_10d4d0e9-4e2d-4d45-ad11-26c200828d2c" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_83ea005b-5e77-47a0-87ee-bb4bcbb083a9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_83860def-69f5-40ad-8f5d-124c25d33983" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_83ea005b-5e77-47a0-87ee-bb4bcbb083a9" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_83860def-69f5-40ad-8f5d-124c25d33983" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_83860def-69f5-40ad-8f5d-124c25d33983_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_83860def-69f5-40ad-8f5d-124c25d33983" xlink:to="loc_us-gaap_EquityComponentDomain_83860def-69f5-40ad-8f5d-124c25d33983_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_d7684fa3-7047-4bf2-b572-6dc745201563" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_83860def-69f5-40ad-8f5d-124c25d33983" xlink:to="loc_us-gaap_EquityComponentDomain_d7684fa3-7047-4bf2-b572-6dc745201563" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_ca3571cf-602d-47fc-acb6-ce2c50613537" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d7684fa3-7047-4bf2-b572-6dc745201563" xlink:to="loc_us-gaap_WarrantMember_ca3571cf-602d-47fc-acb6-ce2c50613537" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019c8ac0-13a0-76c6-b0fb-edc9f917d9a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d7684fa3-7047-4bf2-b572-6dc745201563" xlink:to="loc_us-gaap_CommonStockMember_019c8ac0-13a0-76c6-b0fb-edc9f917d9a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_789b1887-7327-4280-b837-a8a026c8aa05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_83ea005b-5e77-47a0-87ee-bb4bcbb083a9" xlink:to="loc_us-gaap_AwardTypeAxis_789b1887-7327-4280-b837-a8a026c8aa05" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_789b1887-7327-4280-b837-a8a026c8aa05_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_789b1887-7327-4280-b837-a8a026c8aa05" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_789b1887-7327-4280-b837-a8a026c8aa05_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_607a0d30-77b1-4c21-a395-b86bdced5ca0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_789b1887-7327-4280-b837-a8a026c8aa05" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_607a0d30-77b1-4c21-a395-b86bdced5ca0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_ed7d4e99-dd9a-46e7-ae20-205c0b4a890d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_607a0d30-77b1-4c21-a395-b86bdced5ca0" xlink:to="loc_us-gaap_StockCompensationPlanMember_ed7d4e99-dd9a-46e7-ae20-205c0b4a890d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_RestrictedStockUnitsUnvestedMember_bcd7a712-58f6-42b8-8898-6b1cc88d7833" xlink:href="rnac-20251231.xsd#rnac_RestrictedStockUnitsUnvestedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_607a0d30-77b1-4c21-a395-b86bdced5ca0" xlink:to="loc_rnac_RestrictedStockUnitsUnvestedMember_bcd7a712-58f6-42b8-8898-6b1cc88d7833" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_7234eac0-a8e8-45a4-a1d1-952851f76a29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_607a0d30-77b1-4c21-a395-b86bdced5ca0" xlink:to="loc_us-gaap_EmployeeStockOptionMember_7234eac0-a8e8-45a4-a1d1-952851f76a29" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_64f4fb06-bc9f-4c74-be05-3ea846dda64b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_607a0d30-77b1-4c21-a395-b86bdced5ca0" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_64f4fb06-bc9f-4c74-be05-3ea846dda64b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_9a04ed8c-9ce7-425e-8ab2-2f98a89dc8cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_607a0d30-77b1-4c21-a395-b86bdced5ca0" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_9a04ed8c-9ce7-425e-8ab2-2f98a89dc8cf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_9924a6b1-7fd8-43b0-912b-71492173aa30" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_10d4d0e9-4e2d-4d45-ad11-26c200828d2c" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_9924a6b1-7fd8-43b0-912b-71492173aa30" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/StockIncentivePlansNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#StockIncentivePlansNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/StockIncentivePlansNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e2f1c0cd-9de8-47ff-a3fc-ee849daaca6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e2f1c0cd-9de8-47ff-a3fc-ee849daaca6e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019bc5e1-a192-7487-9748-360a6f18b622" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e2f1c0cd-9de8-47ff-a3fc-ee849daaca6e" xlink:to="loc_us-gaap_PlanNameAxis_019bc5e1-a192-7487-9748-360a6f18b622" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7487-9748-360a6f18b622_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019bc5e1-a192-7487-9748-360a6f18b622" xlink:to="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7487-9748-360a6f18b622_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7b3e-b4ed-44fd72348e4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019bc5e1-a192-7487-9748-360a6f18b622" xlink:to="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7b3e-b4ed-44fd72348e4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_StockIncentivePlan2016Member_019bc5e1-a192-7acf-b3b8-11b4869fc810" xlink:href="rnac-20251231.xsd#rnac_StockIncentivePlan2016Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7b3e-b4ed-44fd72348e4d" xlink:to="loc_rnac_StockIncentivePlan2016Member_019bc5e1-a192-7acf-b3b8-11b4869fc810" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_EmploymentInducementIncentiveAwardPlanMember_019bc5e1-d660-7028-8b63-5d2fbd33e077" xlink:href="rnac-20251231.xsd#rnac_EmploymentInducementIncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7b3e-b4ed-44fd72348e4d" xlink:to="loc_rnac_EmploymentInducementIncentiveAwardPlanMember_019bc5e1-d660-7028-8b63-5d2fbd33e077" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OldCartesianPlanMember_019bc5e4-2859-759a-a66b-810d6b061fc4" xlink:href="rnac-20251231.xsd#rnac_OldCartesianPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7b3e-b4ed-44fd72348e4d" xlink:to="loc_rnac_OldCartesianPlanMember_019bc5e4-2859-759a-a66b-810d6b061fc4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_April2025PlanMember_019bc5ed-c07d-712d-84f1-3051b14fcda0" xlink:href="rnac-20251231.xsd#rnac_April2025PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7b3e-b4ed-44fd72348e4d" xlink:to="loc_rnac_April2025PlanMember_019bc5ed-c07d-712d-84f1-3051b14fcda0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_October2025PlanMember_019bc5ed-fe5f-71e4-9258-98ba07b7ea5b" xlink:href="rnac-20251231.xsd#rnac_October2025PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019bc5e1-a192-7b3e-b4ed-44fd72348e4d" xlink:to="loc_rnac_October2025PlanMember_019bc5ed-fe5f-71e4-9258-98ba07b7ea5b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019bc5e5-21fb-7204-bc12-1e4dceba5827" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e2f1c0cd-9de8-47ff-a3fc-ee849daaca6e" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019bc5e5-21fb-7204-bc12-1e4dceba5827" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc5e5-21fb-7204-bc12-1e4dceba5827_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019bc5e5-21fb-7204-bc12-1e4dceba5827" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc5e5-21fb-7204-bc12-1e4dceba5827_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc5e5-21fb-76ce-8dff-e1ff423dc8cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019bc5e5-21fb-7204-bc12-1e4dceba5827" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc5e5-21fb-76ce-8dff-e1ff423dc8cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OldCartestianMember_019bc5e5-21fb-7afa-b34a-09e9bf915129" xlink:href="rnac-20251231.xsd#rnac_OldCartestianMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019bc5e5-21fb-76ce-8dff-e1ff423dc8cb" xlink:to="loc_rnac_OldCartestianMember_019bc5e5-21fb-7afa-b34a-09e9bf915129" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019bc5e5-21fb-7ce9-957f-46ee861ee435" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e2f1c0cd-9de8-47ff-a3fc-ee849daaca6e" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019bc5e5-21fb-7ce9-957f-46ee861ee435" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019bc5e5-21fb-7ce9-957f-46ee861ee435_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019bc5e5-21fb-7ce9-957f-46ee861ee435" xlink:to="loc_us-gaap_EquityComponentDomain_019bc5e5-21fb-7ce9-957f-46ee861ee435_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019bc5e5-21fb-7328-ac13-78b8a04da6ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019bc5e5-21fb-7ce9-957f-46ee861ee435" xlink:to="loc_us-gaap_EquityComponentDomain_019bc5e5-21fb-7328-ac13-78b8a04da6ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019bc5e5-21fb-7f74-8b70-356c3ea139f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019bc5e5-21fb-7328-ac13-78b8a04da6ee" xlink:to="loc_us-gaap_CommonStockMember_019bc5e5-21fb-7f74-8b70-356c3ea139f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_019bc5e5-21fb-7676-9a67-2359d777325a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019bc5e5-21fb-7328-ac13-78b8a04da6ee" xlink:to="loc_us-gaap_PreferredStockMember_019bc5e5-21fb-7676-9a67-2359d777325a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019bc5ef-4d93-7e6b-b464-3e3aa67b2792" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e2f1c0cd-9de8-47ff-a3fc-ee849daaca6e" xlink:to="loc_us-gaap_AwardTypeAxis_019bc5ef-4d93-7e6b-b464-3e3aa67b2792" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019bc5ef-4d93-7e6b-b464-3e3aa67b2792_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019bc5ef-4d93-7e6b-b464-3e3aa67b2792" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019bc5ef-4d93-7e6b-b464-3e3aa67b2792_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019bc5ef-4d93-7b79-8acb-256ae3e7b8a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019bc5ef-4d93-7e6b-b464-3e3aa67b2792" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019bc5ef-4d93-7b79-8acb-256ae3e7b8a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019bc5ef-4d93-7533-803d-da44957093a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019bc5ef-4d93-7b79-8acb-256ae3e7b8a5" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019bc5ef-4d93-7533-803d-da44957093a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019bc5f0-ed84-72e9-b16a-b6c35a47a729" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019bc5ef-4d93-7b79-8acb-256ae3e7b8a5" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019bc5f0-ed84-72e9-b16a-b6c35a47a729" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis_019c8ab7-1a73-7a8e-8910-262466bded76" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e2f1c0cd-9de8-47ff-a3fc-ee849daaca6e" xlink:to="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis_019c8ab7-1a73-7a8e-8910-262466bded76" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_019c8ab7-1a73-7a8e-8910-262466bded76_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsForErrorCorrectionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis_019c8ab7-1a73-7a8e-8910-262466bded76" xlink:to="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_019c8ab7-1a73-7a8e-8910-262466bded76_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_019c8ab7-1a73-725c-bde3-19b04f808a31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsForErrorCorrectionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis_019c8ab7-1a73-7a8e-8910-262466bded76" xlink:to="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_019c8ab7-1a73-725c-bde3-19b04f808a31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_StockBasedCompensationExpenseReversalMember_019c8ab7-84c4-72e8-ba5c-cc70c8e6f2e0" xlink:href="rnac-20251231.xsd#rnac_StockBasedCompensationExpenseReversalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_019c8ab7-1a73-725c-bde3-19b04f808a31" xlink:to="loc_rnac_StockBasedCompensationExpenseReversalMember_019c8ab7-84c4-72e8-ba5c-cc70c8e6f2e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019bc5e1-a192-712c-afa4-babab0af7fe3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019bc5e1-a192-712c-afa4-babab0af7fe3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease_019bc5e1-a192-7690-9bc4-324377f718a4" xlink:href="rnac-20251231.xsd#rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease_019bc5e1-a192-7690-9bc4-324377f718a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019bc5e1-a192-75a7-afd5-1e1f3b5d771f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019bc5e1-a192-75a7-afd5-1e1f3b5d771f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019bc5e4-2859-7da2-a544-fb4b613e1b1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019bc5e4-2859-7da2-a544-fb4b613e1b1c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019bc5e4-2859-7ed8-8755-88c1f7728476" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019bc5e4-2859-7ed8-8755-88c1f7728476" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019bc5e5-21fb-799e-938e-46c28aa8f842" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019bc5e5-21fb-799e-938e-46c28aa8f842" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019bc5e5-21fb-7de6-b380-2a560d01f72f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019bc5e5-21fb-7de6-b380-2a560d01f72f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019bc5e5-21fb-7113-b247-8ca2dc9b76c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019bc5e5-21fb-7113-b247-8ca2dc9b76c3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost_019bc5e6-09f2-7465-87c4-26676eb6d4d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost_019bc5e6-09f2-7465-87c4-26676eb6d4d4" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019bc5ee-8502-78be-885e-6bd73dd99c4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019bc5ee-8502-78be-885e-6bd73dd99c4f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019bc5ee-8502-725a-81ad-a416c70de0c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019bc5ee-8502-725a-81ad-a416c70de0c7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019bc5ef-4d93-7981-8658-671d59b8da98" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019bc5ef-4d93-7981-8658-671d59b8da98" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockConvertibleConversionRatio_019c555b-4814-7450-b3ac-8c08d703ce72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockConvertibleConversionRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_PreferredStockConvertibleConversionRatio_019c555b-4814-7450-b3ac-8c08d703ce72" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019bc5f0-ed84-79ab-a516-4c78a9f9da89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019bc5f0-ed84-79ab-a516-4c78a9f9da89" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019bc5f0-ed84-768f-97e6-f5624c07fc40" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019bc5f0-ed84-768f-97e6-f5624c07fc40" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeituresRate_019bc5f0-ed84-7422-a3aa-d42c9118629d" xlink:href="rnac-20251231.xsd#rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeituresRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeituresRate_019bc5f0-ed84-7422-a3aa-d42c9118629d" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019bc5f0-ed84-70a9-a4b2-ea9bda8a5b44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019bc5f0-ed84-70a9-a4b2-ea9bda8a5b44" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019bc5f0-ed84-7889-8aae-054bda789a78" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019bc5f0-ed84-7889-8aae-054bda789a78" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019bc5f0-ed84-7708-b4ff-d72227e5cf18" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019bc5e1-a192-7b6b-931a-081f7e294c46" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019bc5f0-ed84-7708-b4ff-d72227e5cf18" xlink:type="arc" order="18"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/StockIncentivePlansScheduleofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#StockIncentivePlansScheduleofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/StockIncentivePlansScheduleofStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_4cd3a649-3b50-4666-a8b1-2ef0eb707241" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9d4706a1-cd7f-435a-8c7b-2e74c2b02510" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_4cd3a649-3b50-4666-a8b1-2ef0eb707241" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9d4706a1-cd7f-435a-8c7b-2e74c2b02510" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_7a26f47e-5c50-4db9-adb8-d9d572826c03" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9d4706a1-cd7f-435a-8c7b-2e74c2b02510" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_7a26f47e-5c50-4db9-adb8-d9d572826c03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_7a26f47e-5c50-4db9-adb8-d9d572826c03_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_7a26f47e-5c50-4db9-adb8-d9d572826c03" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_7a26f47e-5c50-4db9-adb8-d9d572826c03_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_62d673a5-9999-4d10-bb0c-07883968eda7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_7a26f47e-5c50-4db9-adb8-d9d572826c03" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_62d673a5-9999-4d10-bb0c-07883968eda7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_9b1c45af-e826-407d-bb45-23dbd1dee790" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_62d673a5-9999-4d10-bb0c-07883968eda7" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_9b1c45af-e826-407d-bb45-23dbd1dee790" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_4be4baca-5dc8-4500-b0a8-35c6ebdf6fe0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_62d673a5-9999-4d10-bb0c-07883968eda7" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_4be4baca-5dc8-4500-b0a8-35c6ebdf6fe0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_c2b6a42e-bc6a-4f3c-8425-c653c3e82069" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_4cd3a649-3b50-4666-a8b1-2ef0eb707241" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_c2b6a42e-bc6a-4f3c-8425-c653c3e82069" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/StockIncentivePlansScheduleofWeightedAverageAssumptionsUsedDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#StockIncentivePlansScheduleofWeightedAverageAssumptionsUsedDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/StockIncentivePlansScheduleofWeightedAverageAssumptionsUsedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5cbf0b8d-2b87-4bf2-b541-c26ecd975c3d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_a1c48278-52bf-453f-bf06-250f57ee5d69" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5cbf0b8d-2b87-4bf2-b541-c26ecd975c3d" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_a1c48278-52bf-453f-bf06-250f57ee5d69" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_094a0048-ac2a-4ebe-903f-5c8d58317803" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_a1c48278-52bf-453f-bf06-250f57ee5d69" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_094a0048-ac2a-4ebe-903f-5c8d58317803" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_094a0048-ac2a-4ebe-903f-5c8d58317803_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_094a0048-ac2a-4ebe-903f-5c8d58317803" xlink:to="loc_us-gaap_EquityComponentDomain_094a0048-ac2a-4ebe-903f-5c8d58317803_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_9aa6b4fa-fad2-4fe1-8e82-5c045f4229c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_094a0048-ac2a-4ebe-903f-5c8d58317803" xlink:to="loc_us-gaap_EquityComponentDomain_9aa6b4fa-fad2-4fe1-8e82-5c045f4229c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_75faee73-e8b6-4919-96e8-1422fae40e83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_9aa6b4fa-fad2-4fe1-8e82-5c045f4229c5" xlink:to="loc_us-gaap_CommonStockMember_75faee73-e8b6-4919-96e8-1422fae40e83" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_3d9cd7af-5537-4041-b337-4533bd8e5d0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_9aa6b4fa-fad2-4fe1-8e82-5c045f4229c5" xlink:to="loc_us-gaap_PreferredStockMember_3d9cd7af-5537-4041-b337-4533bd8e5d0f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_6dd7686a-f96f-4c42-bcf1-50d604dccaf6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_a1c48278-52bf-453f-bf06-250f57ee5d69" xlink:to="loc_us-gaap_AwardTypeAxis_6dd7686a-f96f-4c42-bcf1-50d604dccaf6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6dd7686a-f96f-4c42-bcf1-50d604dccaf6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_6dd7686a-f96f-4c42-bcf1-50d604dccaf6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6dd7686a-f96f-4c42-bcf1-50d604dccaf6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d0d30cea-cc5a-4157-be6a-68e7c15de810" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_6dd7686a-f96f-4c42-bcf1-50d604dccaf6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d0d30cea-cc5a-4157-be6a-68e7c15de810" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_02e8bc36-139c-4da8-8e2b-169bc4e9d6f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d0d30cea-cc5a-4157-be6a-68e7c15de810" xlink:to="loc_us-gaap_EmployeeStockOptionMember_02e8bc36-139c-4da8-8e2b-169bc4e9d6f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_c41b923a-a566-4255-bcac-fcb5dc681a00" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_a1c48278-52bf-453f-bf06-250f57ee5d69" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_c41b923a-a566-4255-bcac-fcb5dc681a00" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c41b923a-a566-4255-bcac-fcb5dc681a00_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_c41b923a-a566-4255-bcac-fcb5dc681a00" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c41b923a-a566-4255-bcac-fcb5dc681a00_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_81db2527-5810-451e-b9f6-2604456c0369" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_c41b923a-a566-4255-bcac-fcb5dc681a00" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_81db2527-5810-451e-b9f6-2604456c0369" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OldCartestianMember_71f309e0-2188-46ef-bff3-a39657e6b4c1" xlink:href="rnac-20251231.xsd#rnac_OldCartestianMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_81db2527-5810-451e-b9f6-2604456c0369" xlink:to="loc_rnac_OldCartestianMember_71f309e0-2188-46ef-bff3-a39657e6b4c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_5b472385-495a-425a-8535-456dc4e75f83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5cbf0b8d-2b87-4bf2-b541-c26ecd975c3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_5b472385-495a-425a-8535-456dc4e75f83" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_ade1ca92-70d0-4813-a447-3784a69584f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5cbf0b8d-2b87-4bf2-b541-c26ecd975c3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_ade1ca92-70d0-4813-a447-3784a69584f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_734ff8ba-3018-46aa-9817-5c8f66f948c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5cbf0b8d-2b87-4bf2-b541-c26ecd975c3d" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_734ff8ba-3018-46aa-9817-5c8f66f948c6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_ec600d15-9f4e-4e5d-a50e-deea6be5a32d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5cbf0b8d-2b87-4bf2-b541-c26ecd975c3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_ec600d15-9f4e-4e5d-a50e-deea6be5a32d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageFairValue_9c2f883f-cffb-472d-9629-344d083c05ea" xlink:href="rnac-20251231.xsd#rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5cbf0b8d-2b87-4bf2-b541-c26ecd975c3d" xlink:to="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageFairValue_9c2f883f-cffb-472d-9629-344d083c05ea" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/StockIncentivePlansScheduleofStockOptionActivityDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#StockIncentivePlansScheduleofStockOptionActivityDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/StockIncentivePlansScheduleofStockOptionActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_50490f35-f84c-41ce-93dc-58edf6a7d698" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_50490f35-f84c-41ce-93dc-58edf6a7d698" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_82ed97c9-c053-47e7-a7d6-320cd0d595a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_50490f35-f84c-41ce-93dc-58edf6a7d698" xlink:to="loc_us-gaap_PlanNameAxis_82ed97c9-c053-47e7-a7d6-320cd0d595a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_82ed97c9-c053-47e7-a7d6-320cd0d595a9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_82ed97c9-c053-47e7-a7d6-320cd0d595a9" xlink:to="loc_us-gaap_PlanNameDomain_82ed97c9-c053-47e7-a7d6-320cd0d595a9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_c53172aa-0471-446a-94f0-f42f95ef6c98" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_82ed97c9-c053-47e7-a7d6-320cd0d595a9" xlink:to="loc_us-gaap_PlanNameDomain_c53172aa-0471-446a-94f0-f42f95ef6c98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_A2008Plan2016PlanAnd2018InducementIncentiveAwardPlanMember_a484949c-9301-4ac9-bb9f-37ed7a7f4d74" xlink:href="rnac-20251231.xsd#rnac_A2008Plan2016PlanAnd2018InducementIncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_c53172aa-0471-446a-94f0-f42f95ef6c98" xlink:to="loc_rnac_A2008Plan2016PlanAnd2018InducementIncentiveAwardPlanMember_a484949c-9301-4ac9-bb9f-37ed7a7f4d74" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_beaeb110-e902-4a75-b106-dd4b92ff14a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_beaeb110-e902-4a75-b106-dd4b92ff14a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_a1a2991b-5944-4790-807f-75d610f9737c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_beaeb110-e902-4a75-b106-dd4b92ff14a8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_a1a2991b-5944-4790-807f-75d610f9737c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_c7899a7f-981b-46d9-9323-f3f53e96a878" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_beaeb110-e902-4a75-b106-dd4b92ff14a8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_c7899a7f-981b-46d9-9323-f3f53e96a878" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsReservedForIssuanceGross_019c2e91-46c9-7e41-8cd4-6ce03bbd7d5a" xlink:href="rnac-20251231.xsd#rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsReservedForIssuanceGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_beaeb110-e902-4a75-b106-dd4b92ff14a8" xlink:to="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsReservedForIssuanceGross_019c2e91-46c9-7e41-8cd4-6ce03bbd7d5a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_1083fa6e-23e9-4ab3-a69c-7766a7974b25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_beaeb110-e902-4a75-b106-dd4b92ff14a8" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_1083fa6e-23e9-4ab3-a69c-7766a7974b25" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_875c5576-f552-4716-8473-04e2d233b6af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_beaeb110-e902-4a75-b106-dd4b92ff14a8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_875c5576-f552-4716-8473-04e2d233b6af" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_a9746ab8-05f9-4a9b-8768-087d1329011a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_bb3f8f2a-f45e-44e1-ab6d-154faed0953d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_bb3f8f2a-f45e-44e1-ab6d-154faed0953d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_a3747ce3-23b8-4ac2-b848-893213486f51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_a3747ce3-23b8-4ac2-b848-893213486f51" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_b4c01554-ad51-4ea5-b253-ffc418e4e6a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_b4c01554-ad51-4ea5-b253-ffc418e4e6a2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_9f3c18cc-3b72-4b7b-8b9f-0e5770cd93c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_b4c01554-ad51-4ea5-b253-ffc418e4e6a2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_9f3c18cc-3b72-4b7b-8b9f-0e5770cd93c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_c619378f-37c9-4114-93a4-caaf7d39ab44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_b4c01554-ad51-4ea5-b253-ffc418e4e6a2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_c619378f-37c9-4114-93a4-caaf7d39ab44" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsReservedForIssuanceWeightedAverageExercisePrice_019c2e90-3e55-7edc-b322-d3b42619d716" xlink:href="rnac-20251231.xsd#rnac_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsReservedForIssuanceWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_b4c01554-ad51-4ea5-b253-ffc418e4e6a2" xlink:to="loc_rnac_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsReservedForIssuanceWeightedAverageExercisePrice_019c2e90-3e55-7edc-b322-d3b42619d716" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_4f003ce0-9106-4f14-bebb-2c583daa94cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_b4c01554-ad51-4ea5-b253-ffc418e4e6a2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_4f003ce0-9106-4f14-bebb-2c583daa94cc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_9737fa25-278b-44ae-9b65-54e54650a6e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_b4c01554-ad51-4ea5-b253-ffc418e4e6a2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_9737fa25-278b-44ae-9b65-54e54650a6e5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_ab5b682e-7324-4050-9077-fbd272161557" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePrice_183f7d06-5723-4324-998e-bdc082a6e83a" xlink:href="rnac-20251231.xsd#rnac_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_rnac_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePrice_183f7d06-5723-4324-998e-bdc082a6e83a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_bb230d52-f51b-4869-9499-f4b247c16e10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_bb230d52-f51b-4869-9499-f4b247c16e10" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_58ae13ee-12b6-4152-8bee-3c71b9121025" xlink:href="rnac-20251231.xsd#rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_58ae13ee-12b6-4152-8bee-3c71b9121025" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_01704e5b-e820-41ee-af73-3ddeafa4f30d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_58ae13ee-12b6-4152-8bee-3c71b9121025" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_01704e5b-e820-41ee-af73-3ddeafa4f30d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm_6f6c6b3d-c528-4d3c-b06d-6806e906e062" xlink:href="rnac-20251231.xsd#rnac_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_58ae13ee-12b6-4152-8bee-3c71b9121025" xlink:to="loc_rnac_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm_6f6c6b3d-c528-4d3c-b06d-6806e906e062" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_6374e4b0-e44e-4687-b04f-9641d026784e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_58ae13ee-12b6-4152-8bee-3c71b9121025" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_6374e4b0-e44e-4687-b04f-9641d026784e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_72f7c56d-8ead-45e8-b0da-564ed09c0892" xlink:href="rnac-20251231.xsd#rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_bc74dbbe-9249-4be6-9516-43b6251b7628" xlink:to="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_72f7c56d-8ead-45e8-b0da-564ed09c0892" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_30375ff8-8fd7-4b82-9b13-020409713d8f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_72f7c56d-8ead-45e8-b0da-564ed09c0892" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_30375ff8-8fd7-4b82-9b13-020409713d8f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingIntrinsicValue_fdc19879-ed90-407d-8431-1d3da3989f26" xlink:href="rnac-20251231.xsd#rnac_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_72f7c56d-8ead-45e8-b0da-564ed09c0892" xlink:to="loc_rnac_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingIntrinsicValue_fdc19879-ed90-407d-8431-1d3da3989f26" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_d8412dc3-8c8b-4ab5-b19f-8495c24026e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_rnac_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_72f7c56d-8ead-45e8-b0da-564ed09c0892" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_d8412dc3-8c8b-4ab5-b19f-8495c24026e8" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/StockIncentivePlansScheduleofRestrictedStockUnitsActivityDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#StockIncentivePlansScheduleofRestrictedStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/StockIncentivePlansScheduleofRestrictedStockUnitsActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_8988489f-c7a3-47c2-aa76-1e99fb01d32b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5a30d93f-44aa-40ca-8abe-9c1b6a423dcd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_8988489f-c7a3-47c2-aa76-1e99fb01d32b" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5a30d93f-44aa-40ca-8abe-9c1b6a423dcd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_dc564e34-86a2-4fb0-93fa-eec1e087177b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5a30d93f-44aa-40ca-8abe-9c1b6a423dcd" xlink:to="loc_us-gaap_AwardTypeAxis_dc564e34-86a2-4fb0-93fa-eec1e087177b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_dc564e34-86a2-4fb0-93fa-eec1e087177b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_dc564e34-86a2-4fb0-93fa-eec1e087177b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_dc564e34-86a2-4fb0-93fa-eec1e087177b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e4011dc2-cd62-4b24-90f1-13c6b4b19f95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_dc564e34-86a2-4fb0-93fa-eec1e087177b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e4011dc2-cd62-4b24-90f1-13c6b4b19f95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_4b18e29f-a4c7-4abd-a516-4d7e6925bfb4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e4011dc2-cd62-4b24-90f1-13c6b4b19f95" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_4b18e29f-a4c7-4abd-a516-4d7e6925bfb4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_69201ad1-7f19-444b-96f1-ad63129a8e05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_8988489f-c7a3-47c2-aa76-1e99fb01d32b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_69201ad1-7f19-444b-96f1-ad63129a8e05" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_7c71902a-d6ed-459c-a3cc-e3b07b5003b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_69201ad1-7f19-444b-96f1-ad63129a8e05" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_7c71902a-d6ed-459c-a3cc-e3b07b5003b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_2d394c3d-5d5b-4133-b318-f81733a923a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_69201ad1-7f19-444b-96f1-ad63129a8e05" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_2d394c3d-5d5b-4133-b318-f81733a923a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019c2e93-4efe-761d-aa70-77ed5b81fa78" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_69201ad1-7f19-444b-96f1-ad63129a8e05" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019c2e93-4efe-761d-aa70-77ed5b81fa78" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_7a615e72-ce7e-4bb7-a1fa-a972cb95a87e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_69201ad1-7f19-444b-96f1-ad63129a8e05" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_7a615e72-ce7e-4bb7-a1fa-a972cb95a87e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_48390811-753c-4e14-be93-4fefa460ea2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ac5617c1-ff72-47be-b112-437ad343326c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_8988489f-c7a3-47c2-aa76-1e99fb01d32b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ac5617c1-ff72-47be-b112-437ad343326c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_c45f36f2-7788-442e-b1a6-174db4f19313" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ac5617c1-ff72-47be-b112-437ad343326c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_c45f36f2-7788-442e-b1a6-174db4f19313" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_95f36aec-5805-43a3-8ec3-5ecd06857c96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ac5617c1-ff72-47be-b112-437ad343326c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_95f36aec-5805-43a3-8ec3-5ecd06857c96" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019c2e93-4efe-7cb6-abe9-8a8b63eea2e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ac5617c1-ff72-47be-b112-437ad343326c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019c2e93-4efe-7cb6-abe9-8a8b63eea2e6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_34e6cf6c-218e-40a7-8737-a28cf5f75f56" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ac5617c1-ff72-47be-b112-437ad343326c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_34e6cf6c-218e-40a7-8737-a28cf5f75f56" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_ad9b47da-8885-485c-882f-80bdcebfb57d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/RevenueArrangementsNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#RevenueArrangementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/RevenueArrangementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019bc62f-25e2-7792-9881-2bd6d012aa0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019bc62f-25e2-7792-9881-2bd6d012aa0d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019bc62f-25e3-7bd5-8592-e502f5cad8f8" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019bc62f-25e2-7792-9881-2bd6d012aa0d" xlink:to="loc_srt_CounterpartyNameAxis_019bc62f-25e3-7bd5-8592-e502f5cad8f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019bc62f-25e3-7bd5-8592-e502f5cad8f8_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019bc62f-25e3-7bd5-8592-e502f5cad8f8" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019bc62f-25e3-7bd5-8592-e502f5cad8f8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019bc62f-25e3-7869-8e9f-0ca1a74603e5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019bc62f-25e3-7bd5-8592-e502f5cad8f8" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019bc62f-25e3-7869-8e9f-0ca1a74603e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_AudentesTherapeuticsInc.AstellasMember_019bc62f-25e3-7de8-a3bd-6b15cbe29b47" xlink:href="rnac-20251231.xsd#rnac_AudentesTherapeuticsInc.AstellasMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019bc62f-25e3-7869-8e9f-0ca1a74603e5" xlink:to="loc_rnac_AudentesTherapeuticsInc.AstellasMember_019bc62f-25e3-7de8-a3bd-6b15cbe29b47" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SwedishOrphanBiovitrumABSOBIMember_019bc62f-b743-7707-9e92-b3f8ae3efcbe" xlink:href="rnac-20251231.xsd#rnac_SwedishOrphanBiovitrumABSOBIMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019bc62f-25e3-7869-8e9f-0ca1a74603e5" xlink:to="loc_rnac_SwedishOrphanBiovitrumABSOBIMember_019bc62f-b743-7707-9e92-b3f8ae3efcbe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019bc62f-25e3-708d-b075-f9e4d5e1540e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019bc62f-25e2-7792-9881-2bd6d012aa0d" xlink:to="loc_srt_ProductOrServiceAxis_019bc62f-25e3-708d-b075-f9e4d5e1540e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019bc62f-25e3-708d-b075-f9e4d5e1540e_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019bc62f-25e3-708d-b075-f9e4d5e1540e" xlink:to="loc_srt_ProductsAndServicesDomain_019bc62f-25e3-708d-b075-f9e4d5e1540e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019bc62f-25e3-7085-ab0a-514aa967e723" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019bc62f-25e3-708d-b075-f9e4d5e1540e" xlink:to="loc_srt_ProductsAndServicesDomain_019bc62f-25e3-7085-ab0a-514aa967e723" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LicenseMember_019bc62f-25e3-777e-9ca8-19de2329b6e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LicenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019bc62f-25e3-7085-ab0a-514aa967e723" xlink:to="loc_us-gaap_LicenseMember_019bc62f-25e3-777e-9ca8-19de2329b6e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_019bc62f-eb50-7be4-83dd-e960cb6307a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019bc62f-25e2-7792-9881-2bd6d012aa0d" xlink:to="loc_us-gaap_TypeOfArrangementAxis_019bc62f-eb50-7be4-83dd-e960cb6307a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019bc62f-eb50-7be4-83dd-e960cb6307a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019bc62f-eb50-7be4-83dd-e960cb6307a8" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019bc62f-eb50-7be4-83dd-e960cb6307a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019bc62f-eb50-7ea1-ab17-cd31ee293575" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019bc62f-eb50-7be4-83dd-e960cb6307a8" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019bc62f-eb50-7ea1-ab17-cd31ee293575" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementMember_019bc62f-eb50-7865-bf78-5f67eea2720b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollaborativeArrangementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019bc62f-eb50-7ea1-ab17-cd31ee293575" xlink:to="loc_us-gaap_CollaborativeArrangementMember_019bc62f-eb50-7865-bf78-5f67eea2720b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019bc62f-eb50-79fd-9485-48bcb4c07fd2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019bc62f-25e2-7792-9881-2bd6d012aa0d" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019bc62f-eb50-79fd-9485-48bcb4c07fd2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc62f-eb50-79fd-9485-48bcb4c07fd2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019bc62f-eb50-79fd-9485-48bcb4c07fd2" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc62f-eb50-79fd-9485-48bcb4c07fd2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc62f-eb50-7b11-9251-f74dca935fb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019bc62f-eb50-79fd-9485-48bcb4c07fd2" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc62f-eb50-7b11-9251-f74dca935fb6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SOBIPurchaseAgreementMember_019bc62f-eb50-7a09-9fb4-8b9c31b71516" xlink:href="rnac-20251231.xsd#rnac_SOBIPurchaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019bc62f-eb50-7b11-9251-f74dca935fb6" xlink:to="loc_rnac_SOBIPurchaseAgreementMember_019bc62f-eb50-7a09-9fb4-8b9c31b71516" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_019bc630-25ab-7737-a2ee-6e6dfc5618a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019bc62f-25e2-7792-9881-2bd6d012aa0d" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_019bc630-25ab-7737-a2ee-6e6dfc5618a8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_019bc630-25ab-7737-a2ee-6e6dfc5618a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReceivablesBillingStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_019bc630-25ab-7737-a2ee-6e6dfc5618a8" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_019bc630-25ab-7737-a2ee-6e6dfc5618a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_019bc630-25ab-778b-ab76-ba11e6ccde8c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReceivablesBillingStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_019bc630-25ab-7737-a2ee-6e6dfc5618a8" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_019bc630-25ab-778b-ab76-ba11e6ccde8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BilledRevenuesMember_019bc630-25ab-745f-a5d9-a4269c4d7a5c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BilledRevenuesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_019bc630-25ab-778b-ab76-ba11e6ccde8c" xlink:to="loc_us-gaap_BilledRevenuesMember_019bc630-25ab-745f-a5d9-a4269c4d7a5c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GovernmentAssistanceTypeAxis_019bc630-c117-7d67-a516-b34bb8d83fae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019bc62f-25e2-7792-9881-2bd6d012aa0d" xlink:to="loc_us-gaap_GovernmentAssistanceTypeAxis_019bc630-c117-7d67-a516-b34bb8d83fae" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GovernmentAssistanceTypeDomain_019bc630-c117-7d67-a516-b34bb8d83fae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GovernmentAssistanceTypeAxis_019bc630-c117-7d67-a516-b34bb8d83fae" xlink:to="loc_us-gaap_GovernmentAssistanceTypeDomain_019bc630-c117-7d67-a516-b34bb8d83fae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GovernmentAssistanceTypeDomain_019bc630-c117-7e29-8739-5f91f70fce48" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GovernmentAssistanceTypeAxis_019bc630-c117-7d67-a516-b34bb8d83fae" xlink:to="loc_us-gaap_GovernmentAssistanceTypeDomain_019bc630-c117-7e29-8739-5f91f70fce48" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_NationalInstituteOfNeurologicalDisordersAndStrokeOfTheNationalInstitutesOfHealthFundingMember_019bc630-c117-7bc0-ae2a-8bdff0f571c9" xlink:href="rnac-20251231.xsd#rnac_NationalInstituteOfNeurologicalDisordersAndStrokeOfTheNationalInstitutesOfHealthFundingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GovernmentAssistanceTypeDomain_019bc630-c117-7e29-8739-5f91f70fce48" xlink:to="loc_rnac_NationalInstituteOfNeurologicalDisordersAndStrokeOfTheNationalInstitutesOfHealthFundingMember_019bc630-c117-7bc0-ae2a-8bdff0f571c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_019bc62f-25e3-74de-a5c4-5ba8d5ee5a4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_ContractWithCustomerLiability_019bc62f-25e3-74de-a5c4-5ba8d5ee5a4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_RevenueRemainingPerformanceObligationVariableConsiderationAmount_019bc62f-25e3-7118-b0a7-4e50c68782fb" xlink:href="rnac-20251231.xsd#rnac_RevenueRemainingPerformanceObligationVariableConsiderationAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_RevenueRemainingPerformanceObligationVariableConsiderationAmount_019bc62f-25e3-7118-b0a7-4e50c68782fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ReimbursementPercentageBudgetedCostsIncurredToCompleteTheDevelopment_019bc62f-540c-7318-9c0e-ab7576c99e0c" xlink:href="rnac-20251231.xsd#rnac_ReimbursementPercentageBudgetedCostsIncurredToCompleteTheDevelopment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_ReimbursementPercentageBudgetedCostsIncurredToCompleteTheDevelopment_019bc62f-540c-7318-9c0e-ab7576c99e0c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LicenseAndOptionAgreementDevelopmentCostReimbursements_019bc62f-73c0-7b88-ae08-5c5cdef6faec" xlink:href="rnac-20251231.xsd#rnac_LicenseAndOptionAgreementDevelopmentCostReimbursements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_LicenseAndOptionAgreementDevelopmentCostReimbursements_019bc62f-73c0-7b88-ae08-5c5cdef6faec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LicenseAndOptionAgreementWrittenNoticePeriodBeforeCancellation_019bc62f-73c0-7421-a0fd-f01dd1e4370c" xlink:href="rnac-20251231.xsd#rnac_LicenseAndOptionAgreementWrittenNoticePeriodBeforeCancellation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_LicenseAndOptionAgreementWrittenNoticePeriodBeforeCancellation_019bc62f-73c0-7421-a0fd-f01dd1e4370c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019bc62f-93ac-7186-a1b9-ca04ec9566ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019bc62f-93ac-7186-a1b9-ca04ec9566ab" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019bc62f-93ac-7aab-bb9e-df062b7b4031" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019bc62f-93ac-7aab-bb9e-df062b7b4031" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019bc62f-93ac-7f78-8f64-25463d28e95e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019bc62f-93ac-7f78-8f64-25463d28e95e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice_019bc62f-93ac-713d-973c-4c5b39b406f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice_019bc62f-93ac-713d-973c-4c5b39b406f7" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_EligiblePeriodToReceiveRoyalties_019bc62f-eb50-7779-9963-cd818132f88b" xlink:href="rnac-20251231.xsd#rnac_EligiblePeriodToReceiveRoyalties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_EligiblePeriodToReceiveRoyalties_019bc62f-eb50-7779-9963-cd818132f88b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_RevenueNumberOfPerformanceObligations_019bc62f-eb50-7d33-a880-fcdecd3a9f16" xlink:href="rnac-20251231.xsd#rnac_RevenueNumberOfPerformanceObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_RevenueNumberOfPerformanceObligations_019bc62f-eb50-7d33-a880-fcdecd3a9f16" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LicenseAndOptionAgreementUpfrontCashPayment_019bc62f-eb50-7081-95c2-64603c978428" xlink:href="rnac-20251231.xsd#rnac_LicenseAndOptionAgreementUpfrontCashPayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_LicenseAndOptionAgreementUpfrontCashPayment_019bc62f-eb50-7081-95c2-64603c978428" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LicenseAndOptionAgreementDevelopmentMilestone_019bc62f-eb50-7bf9-99f3-6b4940eba25c" xlink:href="rnac-20251231.xsd#rnac_LicenseAndOptionAgreementDevelopmentMilestone"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_LicenseAndOptionAgreementDevelopmentMilestone_019bc62f-eb50-7bf9-99f3-6b4940eba25c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019bc62f-eb50-76e1-b9a3-b20168da032a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019bc62f-eb50-76e1-b9a3-b20168da032a" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedemptionPremium_019bc62f-eb50-7ee0-b83d-ff5264550f8f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RedemptionPremium"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_RedemptionPremium_019bc62f-eb50-7ee0-b83d-ff5264550f8f" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_CollaborativeArrangementPaymentObligation_019bc630-25ab-725f-bdd6-37d1a7806d85" xlink:href="rnac-20251231.xsd#rnac_CollaborativeArrangementPaymentObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_CollaborativeArrangementPaymentObligation_019bc630-25ab-725f-bdd6-37d1a7806d85" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019bc630-25ab-797e-9e9a-e19d74581310" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019bc630-25ab-797e-9e9a-e19d74581310" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GovernmentAssistanceAwardAmount_019bc630-c117-73b9-b6b5-e472091f1350" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceAwardAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_GovernmentAssistanceAwardAmount_019bc630-c117-73b9-b6b5-e472091f1350" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_GovernmentAssistancePotentialAdditionalAwardAmount_019bc630-c117-7f77-8034-a31394f6655e" xlink:href="rnac-20251231.xsd#rnac_GovernmentAssistancePotentialAdditionalAwardAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_GovernmentAssistancePotentialAdditionalAwardAmount_019bc630-c117-7f77-8034-a31394f6655e" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_GovernmentAssistanceCurrentStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_019bc630-c117-79a9-adec-e8dfcaad123d" xlink:href="rnac-20251231.xsd#rnac_GovernmentAssistanceCurrentStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_rnac_GovernmentAssistanceCurrentStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_019bc630-c117-79a9-adec-e8dfcaad123d" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GovernmentAssistanceAmountCumulativeCurrent_019bc630-c117-76b8-b4f9-90ceae7aaf21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceAmountCumulativeCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_GovernmentAssistanceAmountCumulativeCurrent_019bc630-c117-76b8-b4f9-90ceae7aaf21" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueNotFromContractWithCustomer_019bc630-c117-76eb-a03f-25954f650b2c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueNotFromContractWithCustomer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019bc62f-25e3-788b-b310-eceaf28247e6" xlink:to="loc_us-gaap_RevenueNotFromContractWithCustomer_019bc630-c117-76eb-a03f-25954f650b2c" xlink:type="arc" order="21"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/RelatedpartyTransactionsScheduleofRelatedPartyTransactionsDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#RelatedpartyTransactionsScheduleofRelatedPartyTransactionsDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/RelatedpartyTransactionsScheduleofRelatedPartyTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_6e98318a-19f3-46bd-9668-bd9971aa2624" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c797d118-de10-45d6-af3a-05f78eb4bcdb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_6e98318a-19f3-46bd-9668-bd9971aa2624" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c797d118-de10-45d6-af3a-05f78eb4bcdb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_b7290692-1ccb-4fb3-a37b-027e4897ab0b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c797d118-de10-45d6-af3a-05f78eb4bcdb" xlink:to="loc_srt_CounterpartyNameAxis_b7290692-1ccb-4fb3-a37b-027e4897ab0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_b7290692-1ccb-4fb3-a37b-027e4897ab0b_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_b7290692-1ccb-4fb3-a37b-027e4897ab0b" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_b7290692-1ccb-4fb3-a37b-027e4897ab0b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_52581bc0-8a17-4b7e-9b20-829845f79bad" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_b7290692-1ccb-4fb3-a37b-027e4897ab0b" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_52581bc0-8a17-4b7e-9b20-829845f79bad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_Dr.TimothyA.SpringerMember_abbd1e5f-5e0e-4eae-8657-8fd104d296c3" xlink:href="rnac-20251231.xsd#rnac_Dr.TimothyA.SpringerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_52581bc0-8a17-4b7e-9b20-829845f79bad" xlink:to="loc_rnac_Dr.TimothyA.SpringerMember_abbd1e5f-5e0e-4eae-8657-8fd104d296c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_TASPartnersLLCMember_35beae8a-9764-44b4-a866-33d0f44ccaa5" xlink:href="rnac-20251231.xsd#rnac_TASPartnersLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_52581bc0-8a17-4b7e-9b20-829845f79bad" xlink:to="loc_rnac_TASPartnersLLCMember_35beae8a-9764-44b4-a866-33d0f44ccaa5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_Dr.ChafenLuMember_fa1c6dce-3a9c-48d1-9c28-0d62a236a7e5" xlink:href="rnac-20251231.xsd#rnac_Dr.ChafenLuMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_52581bc0-8a17-4b7e-9b20-829845f79bad" xlink:to="loc_rnac_Dr.ChafenLuMember_fa1c6dce-3a9c-48d1-9c28-0d62a236a7e5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_50101486-ae83-4504-86f5-f435b21d8261" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c797d118-de10-45d6-af3a-05f78eb4bcdb" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_50101486-ae83-4504-86f5-f435b21d8261" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_50101486-ae83-4504-86f5-f435b21d8261_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_50101486-ae83-4504-86f5-f435b21d8261" xlink:to="loc_us-gaap_RelatedPartyDomain_50101486-ae83-4504-86f5-f435b21d8261_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_f1da9941-ee96-4d93-9741-eeaa5b99ef1d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_50101486-ae83-4504-86f5-f435b21d8261" xlink:to="loc_us-gaap_RelatedPartyDomain_f1da9941-ee96-4d93-9741-eeaa5b99ef1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_ef1d2b2e-263b-4934-92c5-911eb8fb0a5f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_f1da9941-ee96-4d93-9741-eeaa5b99ef1d" xlink:to="loc_us-gaap_RelatedPartyMember_ef1d2b2e-263b-4934-92c5-911eb8fb0a5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_93e1e09e-c6e8-49fd-bbc7-d19361391602" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c797d118-de10-45d6-af3a-05f78eb4bcdb" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_93e1e09e-c6e8-49fd-bbc7-d19361391602" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_93e1e09e-c6e8-49fd-bbc7-d19361391602_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_93e1e09e-c6e8-49fd-bbc7-d19361391602" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_93e1e09e-c6e8-49fd-bbc7-d19361391602_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_502dcba4-607f-403b-bf13-1e6c5253484c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_93e1e09e-c6e8-49fd-bbc7-d19361391602" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_502dcba4-607f-403b-bf13-1e6c5253484c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_July2024PurchaseAgreementMember_35f85c9b-e672-4841-8807-f5cb70f06f71" xlink:href="rnac-20251231.xsd#rnac_July2024PurchaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_502dcba4-607f-403b-bf13-1e6c5253484c" xlink:to="loc_rnac_July2024PurchaseAgreementMember_35f85c9b-e672-4841-8807-f5cb70f06f71" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_PrivatePlacement2023Member_b5139c26-d739-479f-96cd-7cd535ee4bc9" xlink:href="rnac-20251231.xsd#rnac_PrivatePlacement2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_502dcba4-607f-403b-bf13-1e6c5253484c" xlink:to="loc_rnac_PrivatePlacement2023Member_b5139c26-d739-479f-96cd-7cd535ee4bc9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_7cebbd3d-6ca1-40a9-a21b-53eee1b22375" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_6e98318a-19f3-46bd-9668-bd9971aa2624" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_7cebbd3d-6ca1-40a9-a21b-53eee1b22375" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_f6468cb4-07ee-4c89-9f84-bc783612905a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_6e98318a-19f3-46bd-9668-bd9971aa2624" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_f6468cb4-07ee-4c89-9f84-bc783612905a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/RelatedPartyTransactionsNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#RelatedPartyTransactionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/RelatedPartyTransactionsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_37e6edf5-3dd1-4345-bf5b-ef5a2cca5d5d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_37e6edf5-3dd1-4345-bf5b-ef5a2cca5d5d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_1821e4c3-4ce1-47d6-9ed0-346ccfcf79fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_37e6edf5-3dd1-4345-bf5b-ef5a2cca5d5d" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_1821e4c3-4ce1-47d6-9ed0-346ccfcf79fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_1821e4c3-4ce1-47d6-9ed0-346ccfcf79fe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_1821e4c3-4ce1-47d6-9ed0-346ccfcf79fe" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_1821e4c3-4ce1-47d6-9ed0-346ccfcf79fe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_fef0a8f3-2afe-4ff8-8037-b6ae8fab14d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_1821e4c3-4ce1-47d6-9ed0-346ccfcf79fe" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_fef0a8f3-2afe-4ff8-8037-b6ae8fab14d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_SecuritiesPurchaseAgreement2023Member_82154a37-b623-4ed8-91d2-ed6614a43165" xlink:href="rnac-20251231.xsd#rnac_SecuritiesPurchaseAgreement2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_fef0a8f3-2afe-4ff8-8037-b6ae8fab14d3" xlink:to="loc_rnac_SecuritiesPurchaseAgreement2023Member_82154a37-b623-4ed8-91d2-ed6614a43165" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b8fb29e3-c183-4aa1-bdde-842f7cb4f17a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_37e6edf5-3dd1-4345-bf5b-ef5a2cca5d5d" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b8fb29e3-c183-4aa1-bdde-842f7cb4f17a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_b8fb29e3-c183-4aa1-bdde-842f7cb4f17a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b8fb29e3-c183-4aa1-bdde-842f7cb4f17a" xlink:to="loc_us-gaap_RelatedPartyDomain_b8fb29e3-c183-4aa1-bdde-842f7cb4f17a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_8e83bb52-70ba-4ca0-873f-36a6b3556168" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b8fb29e3-c183-4aa1-bdde-842f7cb4f17a" xlink:to="loc_us-gaap_RelatedPartyDomain_8e83bb52-70ba-4ca0-873f-36a6b3556168" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_15df7978-0e5f-4c26-87cf-9d90a06bb983" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_8e83bb52-70ba-4ca0-873f-36a6b3556168" xlink:to="loc_us-gaap_RelatedPartyMember_15df7978-0e5f-4c26-87cf-9d90a06bb983" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_aedc83cc-857e-4778-aaea-702a41c7c511" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_37e6edf5-3dd1-4345-bf5b-ef5a2cca5d5d" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_aedc83cc-857e-4778-aaea-702a41c7c511" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_aedc83cc-857e-4778-aaea-702a41c7c511_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_aedc83cc-857e-4778-aaea-702a41c7c511" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_aedc83cc-857e-4778-aaea-702a41c7c511_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_c731eabf-3f2b-432d-ab54-f85f5085b76a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_aedc83cc-857e-4778-aaea-702a41c7c511" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_c731eabf-3f2b-432d-ab54-f85f5085b76a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_A2019WarrantsMember_2c5dc51b-7a41-42fc-aacf-7a4e0019cf0c" xlink:href="rnac-20251231.xsd#rnac_A2019WarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_c731eabf-3f2b-432d-ab54-f85f5085b76a" xlink:to="loc_rnac_A2019WarrantsMember_2c5dc51b-7a41-42fc-aacf-7a4e0019cf0c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_a64402b8-4c5f-4379-92f8-324d8e48b675" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_a64402b8-4c5f-4379-92f8-324d8e48b675" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_5149bd90-6b03-4228-ba0b-0ce5bb0079d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_5149bd90-6b03-4228-ba0b-0ce5bb0079d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ClassOfWarrantOrRightExercisedDuringPeriod_1a54c33a-443a-4818-ac17-662fc95c5c5d" xlink:href="rnac-20251231.xsd#rnac_ClassOfWarrantOrRightExercisedDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:to="loc_rnac_ClassOfWarrantOrRightExercisedDuringPeriod_1a54c33a-443a-4818-ac17-662fc95c5c5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_a1721795-4d41-46d7-a49e-b7e622792089" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_a1721795-4d41-46d7-a49e-b7e622792089" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromWarrantExercises_0e89db17-dbfe-4d4c-b2ec-53423c331ae9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromWarrantExercises"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:to="loc_us-gaap_ProceedsFromWarrantExercises_0e89db17-dbfe-4d4c-b2ec-53423c331ae9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_53bc1f0f-06c2-442e-b01a-a45f699eff57" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_53bc1f0f-06c2-442e-b01a-a45f699eff57" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_IssuanceOfContingentValueRightForExerciseOfWarrants_0505746e-e934-4d20-bfaa-919d4227931f" xlink:href="rnac-20251231.xsd#rnac_IssuanceOfContingentValueRightForExerciseOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e4289a6-6180-4517-aa97-6b35e00b39b6" xlink:to="loc_rnac_IssuanceOfContingentValueRightForExerciseOfWarrants_0505746e-e934-4d20-bfaa-919d4227931f" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/CollaborationandLicenseAgreementsBiogenMAIncNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#CollaborationandLicenseAgreementsBiogenMAIncNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/CollaborationandLicenseAgreementsBiogenMAIncNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_2d317ed8-aaf7-4e1d-8f72-9c66c3ad8feb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_b4d91bf3-2d50-42bf-8605-060343a7a3bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_2d317ed8-aaf7-4e1d-8f72-9c66c3ad8feb" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_b4d91bf3-2d50-42bf-8605-060343a7a3bc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_5adbd748-caaa-4b29-80fa-eb8a405bdcc2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_b4d91bf3-2d50-42bf-8605-060343a7a3bc" xlink:to="loc_us-gaap_TypeOfArrangementAxis_5adbd748-caaa-4b29-80fa-eb8a405bdcc2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_5adbd748-caaa-4b29-80fa-eb8a405bdcc2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_5adbd748-caaa-4b29-80fa-eb8a405bdcc2" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_5adbd748-caaa-4b29-80fa-eb8a405bdcc2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_26454d52-4ecc-431f-aa0e-f3ee2e8a42a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_5adbd748-caaa-4b29-80fa-eb8a405bdcc2" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_26454d52-4ecc-431f-aa0e-f3ee2e8a42a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_BiogenM.A.Inc.Member_1bd845e9-d3e6-4e5c-a34c-f1f674ca479c" xlink:href="rnac-20251231.xsd#rnac_BiogenM.A.Inc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_26454d52-4ecc-431f-aa0e-f3ee2e8a42a1" xlink:to="loc_rnac_BiogenM.A.Inc.Member_1bd845e9-d3e6-4e5c-a34c-f1f674ca479c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_CollaborativeArrangementWrittenNoticePeriodForTermination_e408a180-b5ba-49f3-825e-1d41f7e6bd2b" xlink:href="rnac-20251231.xsd#rnac_CollaborativeArrangementWrittenNoticePeriodForTermination"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_2d317ed8-aaf7-4e1d-8f72-9c66c3ad8feb" xlink:to="loc_rnac_CollaborativeArrangementWrittenNoticePeriodForTermination_e408a180-b5ba-49f3-825e-1d41f7e6bd2b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/CollaborationandLicenseAgreementsNationalCancerInstituteoftheNationalInstitutesofHealthNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#CollaborationandLicenseAgreementsNationalCancerInstituteoftheNationalInstitutesofHealthNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/CollaborationandLicenseAgreementsNationalCancerInstituteoftheNationalInstitutesofHealthNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_b9bffce2-53f5-47dd-bbb3-9100df707ee6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_78c23b00-8281-4d1f-bf0f-c99b41877c7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_b9bffce2-53f5-47dd-bbb3-9100df707ee6" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_78c23b00-8281-4d1f-bf0f-c99b41877c7a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_abde8b94-6818-47d1-af2c-ec90a335153a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_78c23b00-8281-4d1f-bf0f-c99b41877c7a" xlink:to="loc_us-gaap_TypeOfArrangementAxis_abde8b94-6818-47d1-af2c-ec90a335153a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_abde8b94-6818-47d1-af2c-ec90a335153a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_abde8b94-6818-47d1-af2c-ec90a335153a" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_abde8b94-6818-47d1-af2c-ec90a335153a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_ef574a10-5b60-468b-af6d-b067fb166788" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_abde8b94-6818-47d1-af2c-ec90a335153a" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_ef574a10-5b60-468b-af6d-b067fb166788" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_NationalCancerInstituteAgreementMember_309f0f1c-55bc-47c2-a5b2-b7ded4634b34" xlink:href="rnac-20251231.xsd#rnac_NationalCancerInstituteAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_ef574a10-5b60-468b-af6d-b067fb166788" xlink:to="loc_rnac_NationalCancerInstituteAgreementMember_309f0f1c-55bc-47c2-a5b2-b7ded4634b34" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LicenseAndOptionAgreementRoyaltyPayment_bb9e6dae-a96f-4a41-971e-c6debed950b2" xlink:href="rnac-20251231.xsd#rnac_LicenseAndOptionAgreementRoyaltyPayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_b9bffce2-53f5-47dd-bbb3-9100df707ee6" xlink:to="loc_rnac_LicenseAndOptionAgreementRoyaltyPayment_bb9e6dae-a96f-4a41-971e-c6debed950b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LicenseAndOptionAgreementBenchmarkRoyaltiesMaximum_a0a78aa6-3625-4e6e-97bb-7dd0d80b9493" xlink:href="rnac-20251231.xsd#rnac_LicenseAndOptionAgreementBenchmarkRoyaltiesMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_b9bffce2-53f5-47dd-bbb3-9100df707ee6" xlink:to="loc_rnac_LicenseAndOptionAgreementBenchmarkRoyaltiesMaximum_a0a78aa6-3625-4e6e-97bb-7dd0d80b9493" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_CollaborativeArrangementWrittenNoticePeriodForTermination_dfb07975-5102-44af-a455-03de578276fc" xlink:href="rnac-20251231.xsd#rnac_CollaborativeArrangementWrittenNoticePeriodForTermination"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_b9bffce2-53f5-47dd-bbb3-9100df707ee6" xlink:to="loc_rnac_CollaborativeArrangementWrittenNoticePeriodForTermination_dfb07975-5102-44af-a455-03de578276fc" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/CollaborationandLicenseAgreementsGenovisABpublNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#CollaborationandLicenseAgreementsGenovisABpublNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/CollaborationandLicenseAgreementsGenovisABpublNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_1b1e6ead-3d81-4a6e-b5cb-46d80d96409d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_b982de98-52cd-49ab-8dd5-e1d2d84c9cc0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_1b1e6ead-3d81-4a6e-b5cb-46d80d96409d" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_b982de98-52cd-49ab-8dd5-e1d2d84c9cc0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_a9da9186-a953-4cdc-a7be-1f283ea914cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_b982de98-52cd-49ab-8dd5-e1d2d84c9cc0" xlink:to="loc_us-gaap_TypeOfArrangementAxis_a9da9186-a953-4cdc-a7be-1f283ea914cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_a9da9186-a953-4cdc-a7be-1f283ea914cf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_a9da9186-a953-4cdc-a7be-1f283ea914cf" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_a9da9186-a953-4cdc-a7be-1f283ea914cf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_b95a24cb-4b97-4319-bd45-7c987b0deee7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_a9da9186-a953-4cdc-a7be-1f283ea914cf" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_b95a24cb-4b97-4319-bd45-7c987b0deee7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_GenovisABMember_bd45a6b0-81dd-42de-9514-f2af19d731a7" xlink:href="rnac-20251231.xsd#rnac_GenovisABMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_b95a24cb-4b97-4319-bd45-7c987b0deee7" xlink:to="loc_rnac_GenovisABMember_bd45a6b0-81dd-42de-9514-f2af19d731a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LicenseAndOptionAgreementMilestonePayments_b4e340b6-4af3-44ee-b122-f7fbc39f9600" xlink:href="rnac-20251231.xsd#rnac_LicenseAndOptionAgreementMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_1b1e6ead-3d81-4a6e-b5cb-46d80d96409d" xlink:to="loc_rnac_LicenseAndOptionAgreementMilestonePayments_b4e340b6-4af3-44ee-b122-f7fbc39f9600" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/CollaborationandLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#CollaborationandLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/CollaborationandLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_c1141f36-c376-4ab4-bb96-dd501e680d3f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_099dbf56-dffb-40c9-9fe8-5bbb6819f878" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_c1141f36-c376-4ab4-bb96-dd501e680d3f" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_099dbf56-dffb-40c9-9fe8-5bbb6819f878" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_af0d4bed-91b3-457a-92cf-9f9c9dd2b030" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_099dbf56-dffb-40c9-9fe8-5bbb6819f878" xlink:to="loc_us-gaap_TypeOfArrangementAxis_af0d4bed-91b3-457a-92cf-9f9c9dd2b030" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_af0d4bed-91b3-457a-92cf-9f9c9dd2b030_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_af0d4bed-91b3-457a-92cf-9f9c9dd2b030" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_af0d4bed-91b3-457a-92cf-9f9c9dd2b030_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_f8550ae7-2e29-4f80-ac6f-d0a9d2285402" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_af0d4bed-91b3-457a-92cf-9f9c9dd2b030" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_f8550ae7-2e29-4f80-ac6f-d0a9d2285402" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LicenseAgreementWithShenyangSunshinePharmaceuticalCompanyLimitedMember_f9c527f1-0b27-4a75-a607-d077384bcd17" xlink:href="rnac-20251231.xsd#rnac_LicenseAgreementWithShenyangSunshinePharmaceuticalCompanyLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_f8550ae7-2e29-4f80-ac6f-d0a9d2285402" xlink:to="loc_rnac_LicenseAgreementWithShenyangSunshinePharmaceuticalCompanyLimitedMember_f9c527f1-0b27-4a75-a607-d077384bcd17" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid_5939cf42-5146-44ef-a2c2-51b789311c4f" xlink:href="rnac-20251231.xsd#rnac_AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_c1141f36-c376-4ab4-bb96-dd501e680d3f" xlink:to="loc_rnac_AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid_5939cf42-5146-44ef-a2c2-51b789311c4f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsContainingImmTORPlatform_ebf36d36-46b4-40e9-8d57-74c38fc9af18" xlink:href="rnac-20251231.xsd#rnac_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsContainingImmTORPlatform"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_c1141f36-c376-4ab4-bb96-dd501e680d3f" xlink:to="loc_rnac_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsContainingImmTORPlatform_ebf36d36-46b4-40e9-8d57-74c38fc9af18" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_2bed4b05-53b2-4b2f-8ce0-c29efb5b6810" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_b8d9a74a-92f0-4b25-b38f-6942dbc7c304" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwardsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_2bed4b05-53b2-4b2f-8ce0-c29efb5b6810" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_b8d9a74a-92f0-4b25-b38f-6942dbc7c304" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_ac7491ec-f819-4581-b261-cb80e0959eb0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_b8d9a74a-92f0-4b25-b38f-6942dbc7c304" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_ac7491ec-f819-4581-b261-cb80e0959eb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_ac7491ec-f819-4581-b261-cb80e0959eb0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_ac7491ec-f819-4581-b261-cb80e0959eb0" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_ac7491ec-f819-4581-b261-cb80e0959eb0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_a4414090-16d7-418e-abcb-a438b9c7b9a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_ac7491ec-f819-4581-b261-cb80e0959eb0" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_a4414090-16d7-418e-abcb-a438b9c7b9a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_cf9f8aa9-741d-4159-a67f-a5808ace1d1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_a4414090-16d7-418e-abcb-a438b9c7b9a9" xlink:to="loc_us-gaap_DomesticCountryMember_cf9f8aa9-741d-4159-a67f-a5808ace1d1a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_0ca25424-dab6-4dec-b7ca-f05a847109d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_a4414090-16d7-418e-abcb-a438b9c7b9a9" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_0ca25424-dab6-4dec-b7ca-f05a847109d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019c913f-2e64-7cf8-a9a3-c2506dbde102" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_2bed4b05-53b2-4b2f-8ce0-c29efb5b6810" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019c913f-2e64-7cf8-a9a3-c2506dbde102" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_e6281003-4799-4186-84b2-a9916e504610" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_2bed4b05-53b2-4b2f-8ce0-c29efb5b6810" xlink:to="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_e6281003-4799-4186-84b2-a9916e504610" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_24ad65f1-9634-4ad3-aa87-788ff4159999" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_2bed4b05-53b2-4b2f-8ce0-c29efb5b6810" xlink:to="loc_us-gaap_OperatingLossCarryforwards_24ad65f1-9634-4ad3-aa87-788ff4159999" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OperatingLossCarryforwardsNotSubjectToExpiration_04dfc23f-e9de-40d9-a800-16acb375582f" xlink:href="rnac-20251231.xsd#rnac_OperatingLossCarryforwardsNotSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_2bed4b05-53b2-4b2f-8ce0-c29efb5b6810" xlink:to="loc_rnac_OperatingLossCarryforwardsNotSubjectToExpiration_04dfc23f-e9de-40d9-a800-16acb375582f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OperatingLossCarryforwardsSubjectToExpiration_f0319a6c-eae0-4ac3-aa48-6c901e24ee87" xlink:href="rnac-20251231.xsd#rnac_OperatingLossCarryforwardsSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_2bed4b05-53b2-4b2f-8ce0-c29efb5b6810" xlink:to="loc_rnac_OperatingLossCarryforwardsSubjectToExpiration_f0319a6c-eae0-4ac3-aa48-6c901e24ee87" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAmount_34920360-5820-4713-b47c-f72c95769348" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_2bed4b05-53b2-4b2f-8ce0-c29efb5b6810" xlink:to="loc_us-gaap_TaxCreditCarryforwardAmount_34920360-5820-4713-b47c-f72c95769348" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/RestructuringNarrativeDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#RestructuringNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/RestructuringNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_095eddd7-6b17-41d4-a85e-2465f014e760" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_a9f76239-9118-4dd1-868a-518d5e1acb5c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_095eddd7-6b17-41d4-a85e-2465f014e760" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_a9f76239-9118-4dd1-868a-518d5e1acb5c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_20a0b815-fbdb-4924-88fe-86a68d92326d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_a9f76239-9118-4dd1-868a-518d5e1acb5c" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_20a0b815-fbdb-4924-88fe-86a68d92326d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_20a0b815-fbdb-4924-88fe-86a68d92326d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_20a0b815-fbdb-4924-88fe-86a68d92326d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_20a0b815-fbdb-4924-88fe-86a68d92326d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_16e00ae1-e65f-4e69-9594-99c04a6dfdbd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_20a0b815-fbdb-4924-88fe-86a68d92326d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_16e00ae1-e65f-4e69-9594-99c04a6dfdbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_b5b62c16-607a-46bf-b0ad-f1097deecb49" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_16e00ae1-e65f-4e69-9594-99c04a6dfdbd" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_b5b62c16-607a-46bf-b0ad-f1097deecb49" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_d0801b91-e6e1-4752-8cd1-ee45df6db7f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_16e00ae1-e65f-4e69-9594-99c04a6dfdbd" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_d0801b91-e6e1-4752-8cd1-ee45df6db7f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_36f7dcf2-cefc-4dc2-afc0-f6d790c34736" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_095eddd7-6b17-41d4-a85e-2465f014e760" xlink:to="loc_us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_36f7dcf2-cefc-4dc2-afc0-f6d790c34736" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_740292a6-7828-4731-a78d-9c45e06bd560" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_095eddd7-6b17-41d4-a85e-2465f014e760" xlink:to="loc_us-gaap_RestructuringCharges_740292a6-7828-4731-a78d-9c45e06bd560" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_019bb471-b68d-7f44-932c-46a17e89bcc4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCharges_740292a6-7828-4731-a78d-9c45e06bd560" xlink:to="loc_us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_019bb471-b68d-7f44-932c-46a17e89bcc4" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.cartesiantherapeutics.com/role/SegmentReportingDetails" xlink:type="simple" xlink:href="rnac-20251231.xsd#SegmentReportingDetails"/>
  <link:definitionLink xlink:role="http://www.cartesiantherapeutics.com/role/SegmentReportingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_2e8321e4-4e1d-4e0c-a0a0-c1f8089ff050" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ea9662a2-8696-435e-aa99-bea6e8150e66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2e8321e4-4e1d-4e0c-a0a0-c1f8089ff050" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ea9662a2-8696-435e-aa99-bea6e8150e66" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_054588b9-3084-47db-94f3-61e531051619" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ea9662a2-8696-435e-aa99-bea6e8150e66" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_054588b9-3084-47db-94f3-61e531051619" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_054588b9-3084-47db-94f3-61e531051619_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_054588b9-3084-47db-94f3-61e531051619" xlink:to="loc_us-gaap_SegmentDomain_054588b9-3084-47db-94f3-61e531051619_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_483907ea-436b-42b0-98bd-b87938d245db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_054588b9-3084-47db-94f3-61e531051619" xlink:to="loc_us-gaap_SegmentDomain_483907ea-436b-42b0-98bd-b87938d245db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_OperatingSegmentMember_cd75115f-3a52-4a66-a753-2285815f5819" xlink:href="rnac-20251231.xsd#rnac_OperatingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_483907ea-436b-42b0-98bd-b87938d245db" xlink:to="loc_rnac_OperatingSegmentMember_cd75115f-3a52-4a66-a753-2285815f5819" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_81a9cab9-b058-483c-a89c-330720d13d7c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ea9662a2-8696-435e-aa99-bea6e8150e66" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_81a9cab9-b058-483c-a89c-330720d13d7c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_81a9cab9-b058-483c-a89c-330720d13d7c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_81a9cab9-b058-483c-a89c-330720d13d7c" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_81a9cab9-b058-483c-a89c-330720d13d7c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_c32c7e75-0f1a-4a40-9c98-5f7b4796c448" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_81a9cab9-b058-483c-a89c-330720d13d7c" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_c32c7e75-0f1a-4a40-9c98-5f7b4796c448" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_LegacySelectaProgramsMember_032d25fa-f3b6-467f-9ae9-08f655349b2d" xlink:href="rnac-20251231.xsd#rnac_LegacySelectaProgramsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_c32c7e75-0f1a-4a40-9c98-5f7b4796c448" xlink:to="loc_rnac_LegacySelectaProgramsMember_032d25fa-f3b6-467f-9ae9-08f655349b2d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_Descartes08ForMGMember_f0f02f82-2c89-4f77-b180-d927b2636a40" xlink:href="rnac-20251231.xsd#rnac_Descartes08ForMGMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_c32c7e75-0f1a-4a40-9c98-5f7b4796c448" xlink:to="loc_rnac_Descartes08ForMGMember_f0f02f82-2c89-4f77-b180-d927b2636a40" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_EarlyStageProgramsMember_e10e2934-638d-4b5c-bba8-b8ac460bf952" xlink:href="rnac-20251231.xsd#rnac_EarlyStageProgramsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_c32c7e75-0f1a-4a40-9c98-5f7b4796c448" xlink:to="loc_rnac_EarlyStageProgramsMember_e10e2934-638d-4b5c-bba8-b8ac460bf952" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ResearchAndDevelopmentEmployeeExpensesMember_adcf06f5-0150-425c-be0b-6f5faaf8034e" xlink:href="rnac-20251231.xsd#rnac_ResearchAndDevelopmentEmployeeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_c32c7e75-0f1a-4a40-9c98-5f7b4796c448" xlink:to="loc_rnac_ResearchAndDevelopmentEmployeeExpensesMember_adcf06f5-0150-425c-be0b-6f5faaf8034e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ResearchAndDevelopmentShareBasedPaymentArrangementExpenseMember_d491092a-99df-4169-ac04-3f34e16dd178" xlink:href="rnac-20251231.xsd#rnac_ResearchAndDevelopmentShareBasedPaymentArrangementExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_c32c7e75-0f1a-4a40-9c98-5f7b4796c448" xlink:to="loc_rnac_ResearchAndDevelopmentShareBasedPaymentArrangementExpenseMember_d491092a-99df-4169-ac04-3f34e16dd178" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_ResearchAndDevelopmentFacilitiesAndOtherExpensesMember_5fcec868-2f9e-4883-a870-65638beed59d" xlink:href="rnac-20251231.xsd#rnac_ResearchAndDevelopmentFacilitiesAndOtherExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_c32c7e75-0f1a-4a40-9c98-5f7b4796c448" xlink:to="loc_rnac_ResearchAndDevelopmentFacilitiesAndOtherExpensesMember_5fcec868-2f9e-4883-a870-65638beed59d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_3df16b0d-b3be-46b4-a03c-5189be165859" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2e8321e4-4e1d-4e0c-a0a0-c1f8089ff050" xlink:to="loc_us-gaap_RevenuesAbstract_3df16b0d-b3be-46b4-a03c-5189be165859" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_a157dda8-e412-49e0-8eb5-fc2e079858f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_3df16b0d-b3be-46b4-a03c-5189be165859" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_a157dda8-e412-49e0-8eb5-fc2e079858f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueNotFromContractWithCustomer_d831adaf-6f4e-48ac-bcb7-38840c24e5e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueNotFromContractWithCustomer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_3df16b0d-b3be-46b4-a03c-5189be165859" xlink:to="loc_us-gaap_RevenueNotFromContractWithCustomer_d831adaf-6f4e-48ac-bcb7-38840c24e5e2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_cb440257-9aaa-4f99-816c-0b42bcba7a96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_3df16b0d-b3be-46b4-a03c-5189be165859" xlink:to="loc_us-gaap_Revenues_cb440257-9aaa-4f99-816c-0b42bcba7a96" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_308b72b3-97c2-4668-8cb8-6c7d6d4fec35" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2e8321e4-4e1d-4e0c-a0a0-c1f8089ff050" xlink:to="loc_us-gaap_OperatingExpensesAbstract_308b72b3-97c2-4668-8cb8-6c7d6d4fec35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_e0b3e87c-bdfc-426b-ba09-5fc5a1a15d65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_308b72b3-97c2-4668-8cb8-6c7d6d4fec35" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_e0b3e87c-bdfc-426b-ba09-5fc5a1a15d65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_fd008a6a-c226-4d7a-920c-bb44151856a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_308b72b3-97c2-4668-8cb8-6c7d6d4fec35" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_fd008a6a-c226-4d7a-920c-bb44151856a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_ec93c7de-bc52-4176-a229-9ef10b6924e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfIntangibleAssetsFinitelived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_308b72b3-97c2-4668-8cb8-6c7d6d4fec35" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_ec93c7de-bc52-4176-a229-9ef10b6924e9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_rnac_NonoperatingIncomeExpenseAdjsuted_b6eadb83-dcf3-449a-adf6-7aef186167f3" xlink:href="rnac-20251231.xsd#rnac_NonoperatingIncomeExpenseAdjsuted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2e8321e4-4e1d-4e0c-a0a0-c1f8089ff050" xlink:to="loc_rnac_NonoperatingIncomeExpenseAdjsuted_b6eadb83-dcf3-449a-adf6-7aef186167f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_9cf18758-72c9-449e-8a2a-4e9b1bd6bfaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_2e8321e4-4e1d-4e0c-a0a0-c1f8089ff050" xlink:to="loc_us-gaap_NetIncomeLoss_9cf18758-72c9-449e-8a2a-4e9b1bd6bfaf" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-roles-2025.xsd#eedm"/>
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill" xlink:type="arc" order="691"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense" xlink:type="arc" order="692"/>
  </link:definitionLink>
</link:linkbase>
