<?xml version="1.0" encoding="us-ascii"?>
<!--Generated by Broadridge PROfile 26.3.2.5342 Broadridge-->
<link:linkbase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:t1="http://xbrl.org/2005/xbrldt">
  <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:roleRef roleURI="http://castormaritime.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SubsequentEventsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/SegmentInformationReconciliationOfTotalSegmentAssetsToTotalAssetsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SegmentInformationReconciliationOfTotalSegmentAssetsToTotalAssetsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/SegmentInformationInformationAboutCompanySReportableSegmentsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SegmentInformationInformationAboutCompanySReportableSegmentsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/SharebasedCompensationOptionsActivityDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SharebasedCompensationOptionsActivityDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/SharebasedCompensationAssumptionsForOptionsGrantedDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SharebasedCompensationAssumptionsForOptionsGrantedDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/IncomeTaxesUncertainTaxPositionsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IncomeTaxesUncertainTaxPositionsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/IncomeTaxesComponentsOfDeferredTaxAccountBalancesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IncomeTaxesComponentsOfDeferredTaxAccountBalancesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/IncomeTaxesProvisionForIncomeTaxesExpenseOrBenefitDetailsCalc2" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IncomeTaxesProvisionForIncomeTaxesExpenseOrBenefitDetailsCalc2" />
  <link:roleRef roleURI="http://castormaritime.com/role/IncomeTaxesEffectiveIncomeTaxRateReconciliationDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IncomeTaxesEffectiveIncomeTaxRateReconciliationDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/IncomeTaxesProvisionForIncomeTaxesExpenseOrBenefitDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IncomeTaxesProvisionForIncomeTaxesExpenseOrBenefitDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/InterestAndFinanceCostsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InterestAndFinanceCostsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/CostOfRevenueFromServicesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#CostOfRevenueFromServicesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/GeneralAndAdministrativeExpensesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#GeneralAndAdministrativeExpensesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/VesselOperatingExpensesAndVoyageExpensesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#VesselOperatingExpensesAndVoyageExpensesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/RevenuesRevenueFromServicesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#RevenuesRevenueFromServicesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/RevenuesVesselRevenuesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#RevenuesVesselRevenuesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/EarningsPerCommonShareDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EarningsPerCommonShareDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/CommitmentsAndContingenciesContingenciesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#CommitmentsAndContingenciesContingenciesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/CommitmentsAndContingenciesCommitmentsUnderLongtermLeaseContractsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#CommitmentsAndContingenciesCommitmentsUnderLongtermLeaseContractsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LeasesUndiscountedCashFlowsOfLeaseLiabilityDetailsCalc01" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LeasesUndiscountedCashFlowsOfLeaseLiabilityDetailsCalc01" />
  <link:roleRef roleURI="http://castormaritime.com/role/LeasesUndiscountedCashFlowsOfLeaseLiabilityDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LeasesUndiscountedCashFlowsOfLeaseLiabilityDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LeasesOtherInformationDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LeasesOtherInformationDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LeasesLeaseCostDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LeasesLeaseCostDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosuresRealizedAndUnrealizedGainsAndLossesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#FinancialInstrumentsAndFairValueDisclosuresRealizedAndUnrealizedGainsAndLossesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosuresDerivativeContractsMeasuredOnRecurringBasisDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#FinancialInstrumentsAndFairValueDisclosuresDerivativeContractsMeasuredOnRecurringBasisDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosuresInvestmentInDebtSecuritiesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#FinancialInstrumentsAndFairValueDisclosuresInvestmentInDebtSecuritiesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityCapitalStructureSeriesDCumulativePerpetualConvertiblePreferredSharesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityCapitalStructureSeriesDCumulativePerpetualConvertiblePreferredSharesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityCapitalStructureCapitalStockDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityCapitalStructureCapitalStockDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/InvestmentInDebtSecuritiesCarryingAmountAndUnrealizedGainsOfTradingDebtSecuritiesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InvestmentInDebtSecuritiesCarryingAmountAndUnrealizedGainsOfTradingDebtSecuritiesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/InvestmentInDebtSecuritiesHeldToMaturityInvestmentInDebtSecuritiesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InvestmentInDebtSecuritiesHeldToMaturityInvestmentInDebtSecuritiesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/InvestmentInEquitySecuritiesEquityInvestmentsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InvestmentInEquitySecuritiesEquityInvestmentsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LongtermDebtAndFinancialLiabilitiesAnnualFinancialLiabilitiesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LongtermDebtAndFinancialLiabilitiesAnnualFinancialLiabilitiesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LongtermDebtSaleAndLeasebackAgreementsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LongtermDebtSaleAndLeasebackAgreementsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LongtermDebtAndFinancialLiabilitiesFinancialLiabilitiesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LongtermDebtAndFinancialLiabilitiesFinancialLiabilitiesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LongtermDebtAnnualPrincipalPaymentsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LongtermDebtAnnualPrincipalPaymentsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LongtermDebt500MillionTermLoanFacilityDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LongtermDebt500MillionTermLoanFacilityDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/LongtermDebtLongtermDebtDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LongtermDebtLongtermDebtDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityMethodInvestmentsFairValuesOfAssetsAcquiredAndLiabilitiesAssumedDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityMethodInvestmentsFairValuesOfAssetsAcquiredAndLiabilitiesAssumedDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityMethodInvestmentsSummarizedConsolidatedFinancialInformationDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityMethodInvestmentsSummarizedConsolidatedFinancialInformationDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityMethodInvestmentsEquityMethodInvestmentsMeasuredAtFairValueDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityMethodInvestmentsEquityMethodInvestmentsMeasuredAtFairValueDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityMethodInvestmentsEquityMethodInvestmentsDevelopedDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityMethodInvestmentsEquityMethodInvestmentsDevelopedDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityMethodInvestmentsOwnershipInterestsAndCarryingAmountDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityMethodInvestmentsOwnershipInterestsAndCarryingAmountDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/GoodwillDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#GoodwillDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/IntangibleAssetsNetDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IntangibleAssetsNetDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/PropertyPlantAndEquipmentNetGrossCarryingAmountAndAccumulatedDepreciationDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#PropertyPlantAndEquipmentNetGrossCarryingAmountAndAccumulatedDepreciationDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/PropertyPlantAndEquipmentNetPropertyAndEquipmentByMajorAssetClassesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#PropertyPlantAndEquipmentNetPropertyAndEquipmentByMajorAssetClassesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/VesselsNetVesselAcquisitionsAndOtherCapitalExpendituresDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#VesselsNetVesselAcquisitionsAndOtherCapitalExpendituresDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/VesselsNetNetBookValueDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#VesselsNetNetBookValueDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/FairValueOfAcquiredTimeChartersDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#FairValueOfAcquiredTimeChartersDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/DeferredChargesNetDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#DeferredChargesNetDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedPartiesMpcCapitalRelatedPartiesDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedPartiesMpcCapitalRelatedPartiesDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedPartiesLongtermDebtRelatedPartyDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedPartiesLongtermDebtRelatedPartyDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedPartiesIssuanceOfSeriesDPreferredSharesToToroDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedPartiesIssuanceOfSeriesDPreferredSharesToToroDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedPartiesInvestmentInRelatedPartyDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedPartiesInvestmentInRelatedPartyDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedPartiesPavimarDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedPartiesPavimarDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedPartiesCastorShipsDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedPartiesCastorShipsDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedPartiesSummaryDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedPartiesSummaryDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/BasisOfPresentationAndGeneralInformationGeneralInformationDetails" xlink:type="simple" xlink:href="ctrm-20260630.xsd#BasisOfPresentationAndGeneralInformationGeneralInformationDetails" />
  <link:roleRef roleURI="http://castormaritime.com/role/SegmentInformationTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SegmentInformationTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/SharebasedCompensationTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SharebasedCompensationTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/IncomeTaxesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IncomeTaxesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/InterestAndFinanceCostsTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InterestAndFinanceCostsTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/CostOfRevenueFromServicesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#CostOfRevenueFromServicesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/GeneralAndAdministrativeExpensesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#GeneralAndAdministrativeExpensesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/VesselOperatingExpensesAndVoyageExpensesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#VesselOperatingExpensesAndVoyageExpensesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/RevenuesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#RevenuesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/EarningsPerCommonShareTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EarningsPerCommonShareTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/CommitmentsAndContingenciesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#CommitmentsAndContingenciesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/LeasesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LeasesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosuresTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#FinancialInstrumentsAndFairValueDisclosuresTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/InvestmentInDebtSecuritiesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InvestmentInDebtSecuritiesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/InvestmentInEquitySecuritiesEquityInvestmentsTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InvestmentInEquitySecuritiesEquityInvestmentsTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/LongtermDebtTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LongtermDebtTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityMethodInvestmentsTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityMethodInvestmentsTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/GoodwillTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#GoodwillTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/IntangibleAssetsNetTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IntangibleAssetsNetTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/PropertyPlantAndEquipmentNetTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#PropertyPlantAndEquipmentNetTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/VesselsNetTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#VesselsNetTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/DeferredChargesNetTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#DeferredChargesNetTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedPartiesTables" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedPartiesTables" />
  <link:roleRef roleURI="http://castormaritime.com/role/SignificantAccountingPoliciesAndRecentAccountingPronouncementsPolicies" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SignificantAccountingPoliciesAndRecentAccountingPronouncementsPolicies" />
  <link:roleRef roleURI="http://castormaritime.com/role/BasisOfPresentationAndGeneralInformationPolicies" xlink:type="simple" xlink:href="ctrm-20260630.xsd#BasisOfPresentationAndGeneralInformationPolicies" />
  <link:roleRef roleURI="http://castormaritime.com/role/SubsequentEvents" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SubsequentEvents" />
  <link:roleRef roleURI="http://castormaritime.com/role/SegmentInformation" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SegmentInformation" />
  <link:roleRef roleURI="http://castormaritime.com/role/SharebasedCompensation" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SharebasedCompensation" />
  <link:roleRef roleURI="http://castormaritime.com/role/IncomeTaxes" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IncomeTaxes" />
  <link:roleRef roleURI="http://castormaritime.com/role/InterestAndFinanceCosts" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InterestAndFinanceCosts" />
  <link:roleRef roleURI="http://castormaritime.com/role/CostOfRevenueFromServices" xlink:type="simple" xlink:href="ctrm-20260630.xsd#CostOfRevenueFromServices" />
  <link:roleRef roleURI="http://castormaritime.com/role/GeneralAndAdministrativeExpenses" xlink:type="simple" xlink:href="ctrm-20260630.xsd#GeneralAndAdministrativeExpenses" />
  <link:roleRef roleURI="http://castormaritime.com/role/VesselOperatingExpensesAndVoyageExpenses" xlink:type="simple" xlink:href="ctrm-20260630.xsd#VesselOperatingExpensesAndVoyageExpenses" />
  <link:roleRef roleURI="http://castormaritime.com/role/Revenues" xlink:type="simple" xlink:href="ctrm-20260630.xsd#Revenues" />
  <link:roleRef roleURI="http://castormaritime.com/role/EarningsPerCommonShare" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EarningsPerCommonShare" />
  <link:roleRef roleURI="http://castormaritime.com/role/CommitmentsAndContingencies" xlink:type="simple" xlink:href="ctrm-20260630.xsd#CommitmentsAndContingencies" />
  <link:roleRef roleURI="http://castormaritime.com/role/Leases" xlink:type="simple" xlink:href="ctrm-20260630.xsd#Leases" />
  <link:roleRef roleURI="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosures" xlink:type="simple" xlink:href="ctrm-20260630.xsd#FinancialInstrumentsAndFairValueDisclosures" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityCapitalStructure" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityCapitalStructure" />
  <link:roleRef roleURI="http://castormaritime.com/role/InvestmentInDebtSecurities" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InvestmentInDebtSecurities" />
  <link:roleRef roleURI="http://castormaritime.com/role/InvestmentInEquitySecuritiesEquityInvestments" xlink:type="simple" xlink:href="ctrm-20260630.xsd#InvestmentInEquitySecuritiesEquityInvestments" />
  <link:roleRef roleURI="http://castormaritime.com/role/LongtermDebt" xlink:type="simple" xlink:href="ctrm-20260630.xsd#LongtermDebt" />
  <link:roleRef roleURI="http://castormaritime.com/role/EquityMethodInvestments" xlink:type="simple" xlink:href="ctrm-20260630.xsd#EquityMethodInvestments" />
  <link:roleRef roleURI="http://castormaritime.com/role/Goodwill" xlink:type="simple" xlink:href="ctrm-20260630.xsd#Goodwill" />
  <link:roleRef roleURI="http://castormaritime.com/role/IntangibleAssetsNet" xlink:type="simple" xlink:href="ctrm-20260630.xsd#IntangibleAssetsNet" />
  <link:roleRef roleURI="http://castormaritime.com/role/PropertyPlantAndEquipmentNet" xlink:type="simple" xlink:href="ctrm-20260630.xsd#PropertyPlantAndEquipmentNet" />
  <link:roleRef roleURI="http://castormaritime.com/role/VesselsNet" xlink:type="simple" xlink:href="ctrm-20260630.xsd#VesselsNet" />
  <link:roleRef roleURI="http://castormaritime.com/role/FairValueOfAcquiredTimeCharters" xlink:type="simple" xlink:href="ctrm-20260630.xsd#FairValueOfAcquiredTimeCharters" />
  <link:roleRef roleURI="http://castormaritime.com/role/DeferredChargesNet" xlink:type="simple" xlink:href="ctrm-20260630.xsd#DeferredChargesNet" />
  <link:roleRef roleURI="http://castormaritime.com/role/TransactionsWithRelatedParties" xlink:type="simple" xlink:href="ctrm-20260630.xsd#TransactionsWithRelatedParties" />
  <link:roleRef roleURI="http://castormaritime.com/role/SignificantAccountingPoliciesAndRecentAccountingPronouncements" xlink:type="simple" xlink:href="ctrm-20260630.xsd#SignificantAccountingPoliciesAndRecentAccountingPronouncements" />
  <link:roleRef roleURI="http://castormaritime.com/role/BasisOfPresentationAndGeneralInformation" xlink:type="simple" xlink:href="ctrm-20260630.xsd#BasisOfPresentationAndGeneralInformation" />
  <link:roleRef roleURI="http://castormaritime.com/role/UnauditedInterimCondensedConsolidatedStatementsOfCashFlows" xlink:type="simple" xlink:href="ctrm-20260630.xsd#UnauditedInterimCondensedConsolidatedStatementsOfCashFlows" />
  <link:roleRef roleURI="http://castormaritime.com/role/UnauditedCondensedConsolidatedStatementsOfShareholdersEquityAndMezzanineEquity" xlink:type="simple" xlink:href="ctrm-20260630.xsd#UnauditedCondensedConsolidatedStatementsOfShareholdersEquityAndMezzanineEquity" />
  <link:roleRef roleURI="http://castormaritime.com/role/UnauditedInterimCondensedConsolidatedStatementsOfComprehensiveIncomeParenthetical" xlink:type="simple" xlink:href="ctrm-20260630.xsd#UnauditedInterimCondensedConsolidatedStatementsOfComprehensiveIncomeParenthetical" />
  <link:roleRef roleURI="http://castormaritime.com/role/UnauditedInterimCondensedConsolidatedStatementsOfComprehensiveIncome" xlink:type="simple" xlink:href="ctrm-20260630.xsd#UnauditedInterimCondensedConsolidatedStatementsOfComprehensiveIncome" />
  <link:roleRef roleURI="http://castormaritime.com/role/UnauditedCondensedConsolidatedBalanceSheetsParenthetical" xlink:type="simple" xlink:href="ctrm-20260630.xsd#UnauditedCondensedConsolidatedBalanceSheetsParenthetical" />
  <link:roleRef roleURI="http://castormaritime.com/role/UnauditedCondensedConsolidatedBalanceSheets" xlink:type="simple" xlink:href="ctrm-20260630.xsd#UnauditedCondensedConsolidatedBalanceSheets" />
  <link:roleRef roleURI="http://castormaritime.com/role/DocumentAndEntityInformation" xlink:type="simple" xlink:href="ctrm-20260630.xsd#DocumentAndEntityInformation" />
  <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/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: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:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/DocumentAndEntityInformation" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/UnauditedCondensedConsolidatedBalanceSheets">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="StatementLineItems" xlink:title="StatementLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="AssetsAbstract" xlink:title="AssetsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AssetsAbstract" xlink:title="definition: StatementLineItems to AssetsAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="LiabilitiesAndStockholdersEquityAbstract" xlink:title="LiabilitiesAndStockholdersEquityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="LiabilitiesAndStockholdersEquityAbstract" xlink:title="definition: StatementLineItems to LiabilitiesAndStockholdersEquityAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="LiabilitiesCurrentAbstract" xlink:title="LiabilitiesCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="LiabilitiesCurrentAbstract" xlink:title="definition: StatementLineItems to LiabilitiesCurrentAbstract" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesNoncurrentAbstract" xlink:label="LiabilitiesNoncurrentAbstract" xlink:title="LiabilitiesNoncurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="LiabilitiesNoncurrentAbstract" xlink:title="definition: StatementLineItems to LiabilitiesNoncurrentAbstract" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="CommitmentsAndContingencies" xlink:title="CommitmentsAndContingencies" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CommitmentsAndContingencies" xlink:title="definition: StatementLineItems to CommitmentsAndContingencies" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityAbstract" xlink:label="TemporaryEquityAbstract" xlink:title="TemporaryEquityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquityAbstract" xlink:title="definition: StatementLineItems to TemporaryEquityAbstract" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="StockholdersEquityAbstract" xlink:title="StockholdersEquityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="StockholdersEquityAbstract" xlink:title="definition: StatementLineItems to StockholdersEquityAbstract" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="LiabilitiesAndStockholdersEquity" xlink:title="LiabilitiesAndStockholdersEquity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="LiabilitiesAndStockholdersEquity" xlink:title="definition: StatementLineItems to LiabilitiesAndStockholdersEquity" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue" xlink:label="CommonStockValue" xlink:title="CommonStockValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CommonStockValue" xlink:title="definition: StatementLineItems to CommonStockValue" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue" xlink:label="PreferredStockValue" xlink:title="PreferredStockValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PreferredStockValue" xlink:title="definition: StatementLineItems to PreferredStockValue" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital" xlink:label="AdditionalPaidInCapital" xlink:title="AdditionalPaidInCapital" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AdditionalPaidInCapital" xlink:title="definition: StatementLineItems to AdditionalPaidInCapital" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="RetainedEarningsAccumulatedDeficit" xlink:title="RetainedEarningsAccumulatedDeficit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="RetainedEarningsAccumulatedDeficit" xlink:title="definition: StatementLineItems to RetainedEarningsAccumulatedDeficit" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:title="AccumulatedOtherComprehensiveIncomeLossNetOfTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:title="definition: StatementLineItems to AccumulatedOtherComprehensiveIncomeLossNetOfTax" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity" xlink:label="StockholdersEquity" xlink:title="StockholdersEquity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="StockholdersEquity" xlink:title="definition: StatementLineItems to StockholdersEquity" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterest" xlink:label="MinorityInterest" xlink:title="MinorityInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="MinorityInterest" xlink:title="definition: StatementLineItems to MinorityInterest" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:title="StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:title="definition: StatementLineItems to StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" order="16.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent" xlink:label="TemporaryEquityCarryingAmountAttributableToParent" xlink:title="TemporaryEquityCarryingAmountAttributableToParent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquityCarryingAmountAttributableToParent" xlink:title="definition: StatementLineItems to TemporaryEquityCarryingAmountAttributableToParent" order="17.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredLongTermDebt" xlink:label="SecuredLongTermDebt" xlink:title="SecuredLongTermDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="SecuredLongTermDebt" xlink:title="definition: StatementLineItems to SecuredLongTermDebt" order="18.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLongTermDebtNoncurrent" xlink:label="OtherLongTermDebtNoncurrent" xlink:title="OtherLongTermDebtNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherLongTermDebtNoncurrent" xlink:title="definition: StatementLineItems to OtherLongTermDebtNoncurrent" order="19.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:label="UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:title="UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:title="definition: StatementLineItems to UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" order="20.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAccruedLiabilitiesNoncurrent" xlink:label="OtherAccruedLiabilitiesNoncurrent" xlink:title="OtherAccruedLiabilitiesNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherAccruedLiabilitiesNoncurrent" xlink:title="definition: StatementLineItems to OtherAccruedLiabilitiesNoncurrent" order="21.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent" xlink:label="OperatingLeaseLiabilityNoncurrent" xlink:title="OperatingLeaseLiabilityNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OperatingLeaseLiabilityNoncurrent" xlink:title="definition: StatementLineItems to OperatingLeaseLiabilityNoncurrent" order="22.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet" xlink:label="DeferredIncomeTaxLiabilitiesNet" xlink:title="DeferredIncomeTaxLiabilitiesNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DeferredIncomeTaxLiabilitiesNet" xlink:title="definition: StatementLineItems to DeferredIncomeTaxLiabilitiesNet" order="23.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesNoncurrent" xlink:label="LiabilitiesNoncurrent" xlink:title="LiabilitiesNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="LiabilitiesNoncurrent" xlink:title="definition: StatementLineItems to LiabilitiesNoncurrent" order="24.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtCurrent" xlink:label="SecuredDebtCurrent" xlink:title="SecuredDebtCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="SecuredDebtCurrent" xlink:title="definition: StatementLineItems to SecuredDebtCurrent" order="25.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLongTermDebtCurrent" xlink:label="OtherLongTermDebtCurrent" xlink:title="OtherLongTermDebtCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherLongTermDebtCurrent" xlink:title="definition: StatementLineItems to OtherLongTermDebtCurrent" order="26.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent" xlink:label="AccountsPayableCurrent" xlink:title="AccountsPayableCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AccountsPayableCurrent" xlink:title="definition: StatementLineItems to AccountsPayableCurrent" order="27.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityCurrent" xlink:label="ContractWithCustomerLiabilityCurrent" xlink:title="ContractWithCustomerLiabilityCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ContractWithCustomerLiabilityCurrent" xlink:title="definition: StatementLineItems to ContractWithCustomerLiabilityCurrent" order="28.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:label="AccruedLiabilitiesCurrent" xlink:title="AccruedLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AccruedLiabilitiesCurrent" xlink:title="definition: StatementLineItems to AccruedLiabilitiesCurrent" order="29.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesCurrent" xlink:label="OtherLiabilitiesCurrent" xlink:title="OtherLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherLiabilitiesCurrent" xlink:title="definition: StatementLineItems to OtherLiabilitiesCurrent" order="30.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilityCurrentRelatedPartyTypeExtensibleEnumeration" xlink:label="OtherLiabilityCurrentRelatedPartyTypeExtensibleEnumeration" xlink:title="OtherLiabilityCurrentRelatedPartyTypeExtensibleEnumeration" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherLiabilityCurrentRelatedPartyTypeExtensibleEnumeration" xlink:title="definition: StatementLineItems to OtherLiabilityCurrentRelatedPartyTypeExtensibleEnumeration" order="31.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitiesCurrent" xlink:label="DerivativeLiabilitiesCurrent" xlink:title="DerivativeLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DerivativeLiabilitiesCurrent" xlink:title="definition: StatementLineItems to DerivativeLiabilitiesCurrent" order="32.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent" xlink:label="OperatingLeaseLiabilityCurrent" xlink:title="OperatingLeaseLiabilityCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OperatingLeaseLiabilityCurrent" xlink:title="definition: StatementLineItems to OperatingLeaseLiabilityCurrent" order="33.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedIncomeTaxesCurrent" xlink:label="AccruedIncomeTaxesCurrent" xlink:title="AccruedIncomeTaxesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AccruedIncomeTaxesCurrent" xlink:title="definition: StatementLineItems to AccruedIncomeTaxesCurrent" order="34.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent" xlink:label="LiabilitiesCurrent" xlink:title="LiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="LiabilitiesCurrent" xlink:title="definition: StatementLineItems to LiabilitiesCurrent" order="35.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="AssetsCurrentAbstract" xlink:title="AssetsCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AssetsCurrentAbstract" xlink:title="definition: StatementLineItems to AssetsCurrentAbstract" order="36.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsNoncurrentAbstract" xlink:label="AssetsNoncurrentAbstract" xlink:title="AssetsNoncurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AssetsNoncurrentAbstract" xlink:title="definition: StatementLineItems to AssetsNoncurrentAbstract" order="37.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="Assets" xlink:title="Assets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="Assets" xlink:title="definition: StatementLineItems to Assets" order="38.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="PropertyPlantAndEquipmentNet" xlink:title="PropertyPlantAndEquipmentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PropertyPlantAndEquipmentNet" xlink:title="definition: StatementLineItems to PropertyPlantAndEquipmentNet" order="39.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashNoncurrent" xlink:label="RestrictedCashNoncurrent" xlink:title="RestrictedCashNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="RestrictedCashNoncurrent" xlink:title="definition: StatementLineItems to RestrictedCashNoncurrent" order="40.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:label="OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:title="OtherReceivableAfterAllowanceForCreditLossNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:title="definition: StatementLineItems to OtherReceivableAfterAllowanceForCreditLossNoncurrent" order="41.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivableAfterAllowanceForCreditLossNoncurrentRelatedPartyTypeExtensibleEnumeration" xlink:label="OtherReceivableAfterAllowanceForCreditLossNoncurrentRelatedPartyTypeExtensibleEnumeration" xlink:title="OtherReceivableAfterAllowanceForCreditLossNoncurrentRelatedPartyTypeExtensibleEnumeration" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherReceivableAfterAllowanceForCreditLossNoncurrentRelatedPartyTypeExtensibleEnumeration" xlink:title="definition: StatementLineItems to OtherReceivableAfterAllowanceForCreditLossNoncurrentRelatedPartyTypeExtensibleEnumeration" order="42.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsNoncurrent" xlink:label="PrepaidExpenseAndOtherAssetsNoncurrent" xlink:title="PrepaidExpenseAndOtherAssetsNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PrepaidExpenseAndOtherAssetsNoncurrent" xlink:title="definition: StatementLineItems to PrepaidExpenseAndOtherAssetsNoncurrent" order="43.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostNetNoncurrent" xlink:label="CapitalizedContractCostNetNoncurrent" xlink:title="CapitalizedContractCostNetNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CapitalizedContractCostNetNoncurrent" xlink:title="definition: StatementLineItems to CapitalizedContractCostNetNoncurrent" order="44.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InvestmentInRelatedParty" xlink:label="InvestmentInRelatedParty" xlink:title="InvestmentInRelatedParty" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="InvestmentInRelatedParty" xlink:title="definition: StatementLineItems to InvestmentInRelatedParty" order="45.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityFairValueNoncurrent" xlink:label="DebtSecuritiesHeldToMaturityFairValueNoncurrent" xlink:title="DebtSecuritiesHeldToMaturityFairValueNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DebtSecuritiesHeldToMaturityFairValueNoncurrent" xlink:title="definition: StatementLineItems to DebtSecuritiesHeldToMaturityFairValueNoncurrent" order="46.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments" xlink:label="EquityMethodInvestments" xlink:title="EquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EquityMethodInvestments" xlink:title="definition: StatementLineItems to EquityMethodInvestments" order="47.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentsFairValueDisclosure" xlink:label="EquityMethodInvestmentsFairValueDisclosure" xlink:title="EquityMethodInvestmentsFairValueDisclosure" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EquityMethodInvestmentsFairValueDisclosure" xlink:title="definition: StatementLineItems to EquityMethodInvestmentsFairValueDisclosure" order="48.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" xlink:title="definition: StatementLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" order="49.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill" xlink:label="Goodwill" xlink:title="Goodwill" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="Goodwill" xlink:title="definition: StatementLineItems to Goodwill" order="50.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="FiniteLivedIntangibleAssetsNet" xlink:title="FiniteLivedIntangibleAssetsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="FiniteLivedIntangibleAssetsNet" xlink:title="definition: StatementLineItems to FiniteLivedIntangibleAssetsNet" order="51.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="OperatingLeaseRightOfUseAsset" xlink:title="OperatingLeaseRightOfUseAsset" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OperatingLeaseRightOfUseAsset" xlink:title="definition: StatementLineItems to OperatingLeaseRightOfUseAsset" order="52.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxAssetsNet" xlink:label="DeferredIncomeTaxAssetsNet" xlink:title="DeferredIncomeTaxAssetsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DeferredIncomeTaxAssetsNet" xlink:title="definition: StatementLineItems to DeferredIncomeTaxAssetsNet" order="53.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetsNoncurrent" xlink:label="DerivativeAssetsNoncurrent" xlink:title="DerivativeAssetsNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DerivativeAssetsNoncurrent" xlink:title="definition: StatementLineItems to DerivativeAssetsNoncurrent" order="54.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsNoncurrent" xlink:label="AssetsNoncurrent" xlink:title="AssetsNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AssetsNoncurrent" xlink:title="definition: StatementLineItems to AssetsNoncurrent" order="55.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="CashAndCashEquivalentsAtCarryingValue" xlink:title="CashAndCashEquivalentsAtCarryingValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CashAndCashEquivalentsAtCarryingValue" xlink:title="definition: StatementLineItems to CashAndCashEquivalentsAtCarryingValue" order="56.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="AccountsReceivableNetCurrent" xlink:title="AccountsReceivableNetCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AccountsReceivableNetCurrent" xlink:title="definition: StatementLineItems to AccountsReceivableNetCurrent" order="57.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivablesNetCurrent" xlink:label="OtherReceivablesNetCurrent" xlink:title="OtherReceivablesNetCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherReceivablesNetCurrent" xlink:title="definition: StatementLineItems to OtherReceivablesNetCurrent" order="58.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivableAfterAllowanceForCreditLossCurrentRelatedPartyTypeExtensibleEnumeration" xlink:label="OtherReceivableAfterAllowanceForCreditLossCurrentRelatedPartyTypeExtensibleEnumeration" xlink:title="OtherReceivableAfterAllowanceForCreditLossCurrentRelatedPartyTypeExtensibleEnumeration" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherReceivableAfterAllowanceForCreditLossCurrentRelatedPartyTypeExtensibleEnumeration" xlink:title="definition: StatementLineItems to OtherReceivableAfterAllowanceForCreditLossCurrentRelatedPartyTypeExtensibleEnumeration" order="59.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryNet" xlink:label="InventoryNet" xlink:title="InventoryNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="InventoryNet" xlink:title="definition: StatementLineItems to InventoryNet" order="60.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="PrepaidExpenseAndOtherAssetsCurrent" xlink:title="PrepaidExpenseAndOtherAssetsCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PrepaidExpenseAndOtherAssetsCurrent" xlink:title="definition: StatementLineItems to PrepaidExpenseAndOtherAssetsCurrent" order="61.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesReceivable" xlink:label="IncomeTaxesReceivable" xlink:title="IncomeTaxesReceivable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="IncomeTaxesReceivable" xlink:title="definition: StatementLineItems to IncomeTaxesReceivable" order="62.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNi" xlink:label="EquitySecuritiesFvNi" xlink:title="EquitySecuritiesFvNi" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EquitySecuritiesFvNi" xlink:title="definition: StatementLineItems to EquitySecuritiesFvNi" order="63.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesCurrent" xlink:label="DebtSecuritiesCurrent" xlink:title="DebtSecuritiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DebtSecuritiesCurrent" xlink:title="definition: StatementLineItems to DebtSecuritiesCurrent" order="64.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_AccruedCharterRevenueCurrent" xlink:label="AccruedCharterRevenueCurrent" xlink:title="AccruedCharterRevenueCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AccruedCharterRevenueCurrent" xlink:title="definition: StatementLineItems to AccruedCharterRevenueCurrent" order="65.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetsCurrent" xlink:label="DerivativeAssetsCurrent" xlink:title="DerivativeAssetsCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DerivativeAssetsCurrent" xlink:title="definition: StatementLineItems to DerivativeAssetsCurrent" order="66.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent" xlink:label="AssetsCurrent" xlink:title="AssetsCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="AssetsCurrent" xlink:title="definition: StatementLineItems to AssetsCurrent" order="67.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="StatementTable" xlink:title="StatementTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="StatementLineItems" xlink:to="StatementTable" xlink:title="definition: StatementLineItems to StatementTable" order="68.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="StatementTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: StatementTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VesselsMember" xlink:label="VesselsMember" xlink:title="VesselsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="VesselsMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to VesselsMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyAndEquipmentMember" xlink:label="PropertyAndEquipmentMember" xlink:title="PropertyAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="PropertyAndEquipmentMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to PropertyAndEquipmentMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/UnauditedCondensedConsolidatedBalanceSheetsParenthetical">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="StatementLineItems" xlink:title="StatementLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="LiabilitiesAndStockholdersEquityAbstract" xlink:title="LiabilitiesAndStockholdersEquityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="LiabilitiesAndStockholdersEquityAbstract" xlink:title="definition: StatementLineItems to LiabilitiesAndStockholdersEquityAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityAbstract" xlink:label="TemporaryEquityAbstract" xlink:title="TemporaryEquityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquityAbstract" xlink:title="definition: StatementLineItems to TemporaryEquityAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="StockholdersEquityAbstract" xlink:title="StockholdersEquityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="StockholdersEquityAbstract" xlink:title="definition: StatementLineItems to StockholdersEquityAbstract" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="CommonStockParOrStatedValuePerShare" xlink:title="CommonStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CommonStockParOrStatedValuePerShare" xlink:title="definition: StatementLineItems to CommonStockParOrStatedValuePerShare" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="CommonStockSharesAuthorized" xlink:title="CommonStockSharesAuthorized" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CommonStockSharesAuthorized" xlink:title="definition: StatementLineItems to CommonStockSharesAuthorized" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="CommonStockSharesIssued" xlink:title="CommonStockSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CommonStockSharesIssued" xlink:title="definition: StatementLineItems to CommonStockSharesIssued" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="CommonStockSharesOutstanding" xlink:title="CommonStockSharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CommonStockSharesOutstanding" xlink:title="definition: StatementLineItems to CommonStockSharesOutstanding" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="PreferredStockParOrStatedValuePerShare" xlink:title="PreferredStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PreferredStockParOrStatedValuePerShare" xlink:title="definition: StatementLineItems to PreferredStockParOrStatedValuePerShare" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="PreferredStockSharesAuthorized" xlink:title="PreferredStockSharesAuthorized" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PreferredStockSharesAuthorized" xlink:title="definition: StatementLineItems to PreferredStockSharesAuthorized" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="PreferredStockSharesIssued" xlink:title="PreferredStockSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PreferredStockSharesIssued" xlink:title="definition: StatementLineItems to PreferredStockSharesIssued" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="PreferredStockSharesOutstanding" xlink:title="PreferredStockSharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PreferredStockSharesOutstanding" xlink:title="definition: StatementLineItems to PreferredStockSharesOutstanding" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="PreferredStockDividendRatePercentage" xlink:title="PreferredStockDividendRatePercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PreferredStockDividendRatePercentage" xlink:title="definition: StatementLineItems to PreferredStockDividendRatePercentage" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesIssued" xlink:label="TemporaryEquitySharesIssued" xlink:title="TemporaryEquitySharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquitySharesIssued" xlink:title="definition: StatementLineItems to TemporaryEquitySharesIssued" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesOutstanding" xlink:label="TemporaryEquitySharesOutstanding" xlink:title="TemporaryEquitySharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquitySharesOutstanding" xlink:title="definition: StatementLineItems to TemporaryEquitySharesOutstanding" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityLiquidationPreference" xlink:label="TemporaryEquityLiquidationPreference" xlink:title="TemporaryEquityLiquidationPreference" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquityLiquidationPreference" xlink:title="definition: StatementLineItems to TemporaryEquityLiquidationPreference" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="StatementTable" xlink:title="StatementTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="StatementLineItems" xlink:to="StatementTable" xlink:title="definition: StatementLineItems to StatementTable" order="16.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="StatementClassOfStockAxis" xlink:title="StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="StatementTable" xlink:to="StatementClassOfStockAxis" xlink:title="definition: StatementTable to StatementClassOfStockAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain_2" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain_2" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesDPreferredStockMember" xlink:label="SeriesDPreferredStockMember" xlink:title="SeriesDPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesDPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesDPreferredStockMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember" xlink:label="SeriesBPreferredStockMember" xlink:title="SeriesBPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesBPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesBPreferredStockMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/UnauditedInterimCondensedConsolidatedStatementsOfComprehensiveIncome">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="StatementLineItems" xlink:title="StatementLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesAbstract" xlink:label="RevenuesAbstract" xlink:title="RevenuesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="RevenuesAbstract" xlink:title="definition: StatementLineItems to RevenuesAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostsAndExpensesAbstract" xlink:label="CostsAndExpensesAbstract" xlink:title="CostsAndExpensesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CostsAndExpensesAbstract" xlink:title="definition: StatementLineItems to CostsAndExpensesAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_OtherOperatingIncomeExpenseAbstract" xlink:label="OtherOperatingIncomeExpenseAbstract" xlink:title="OtherOperatingIncomeExpenseAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherOperatingIncomeExpenseAbstract" xlink:title="definition: StatementLineItems to OtherOperatingIncomeExpenseAbstract" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss" xlink:label="OperatingIncomeLoss" xlink:title="OperatingIncomeLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OperatingIncomeLoss" xlink:title="definition: StatementLineItems to OperatingIncomeLoss" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherIncomeAndExpensesAbstract" xlink:label="OtherIncomeAndExpensesAbstract" xlink:title="OtherIncomeAndExpensesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherIncomeAndExpensesAbstract" xlink:title="definition: StatementLineItems to OtherIncomeAndExpensesAbstract" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:label="IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:title="IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:title="definition: StatementLineItems to IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="IncomeTaxExpenseBenefit" xlink:title="IncomeTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="IncomeTaxExpenseBenefit" xlink:title="definition: StatementLineItems to IncomeTaxExpenseBenefit" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="ProfitLoss" xlink:title="ProfitLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ProfitLoss" xlink:title="definition: StatementLineItems to ProfitLoss" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:label="NetIncomeLossAttributableToNoncontrollingInterest" xlink:title="NetIncomeLossAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="NetIncomeLossAttributableToNoncontrollingInterest" xlink:title="definition: StatementLineItems to NetIncomeLossAttributableToNoncontrollingInterest" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="NetIncomeLoss" xlink:title="NetIncomeLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="NetIncomeLoss" xlink:title="definition: StatementLineItems to NetIncomeLoss" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RedeemablePreferredStockDividends" xlink:label="RedeemablePreferredStockDividends" xlink:title="RedeemablePreferredStockDividends" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="RedeemablePreferredStockDividends" xlink:title="definition: StatementLineItems to RedeemablePreferredStockDividends" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:label="PreferredStockDividendsIncomeStatementImpact" xlink:title="PreferredStockDividendsIncomeStatementImpact" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="PreferredStockDividendsIncomeStatementImpact" xlink:title="definition: StatementLineItems to PreferredStockDividendsIncomeStatementImpact" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="NetIncomeLossAvailableToCommonStockholdersBasic" xlink:title="NetIncomeLossAvailableToCommonStockholdersBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="NetIncomeLossAvailableToCommonStockholdersBasic" xlink:title="definition: StatementLineItems to NetIncomeLossAvailableToCommonStockholdersBasic" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:label="OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:title="OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:title="definition: StatementLineItems to OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:label="ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:title="ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:title="definition: StatementLineItems to ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest" xlink:label="ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest" xlink:title="ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest" xlink:title="definition: StatementLineItems to ComprehensiveIncomeNetOfTaxAttributableToNoncontrollingInterest" order="16.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomeNetOfTax" xlink:label="ComprehensiveIncomeNetOfTax" xlink:title="ComprehensiveIncomeNetOfTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ComprehensiveIncomeNetOfTax" xlink:title="definition: StatementLineItems to ComprehensiveIncomeNetOfTax" order="17.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic" xlink:label="EarningsPerShareBasic" xlink:title="EarningsPerShareBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EarningsPerShareBasic" xlink:title="definition: StatementLineItems to EarningsPerShareBasic" order="18.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="EarningsPerShareDiluted" xlink:title="EarningsPerShareDiluted" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EarningsPerShareDiluted" xlink:title="definition: StatementLineItems to EarningsPerShareDiluted" order="19.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="WeightedAverageNumberOfSharesOutstandingBasic" xlink:title="WeightedAverageNumberOfSharesOutstandingBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="WeightedAverageNumberOfSharesOutstandingBasic" xlink:title="definition: StatementLineItems to WeightedAverageNumberOfSharesOutstandingBasic" order="20.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="WeightedAverageNumberOfDilutedSharesOutstanding" xlink:title="WeightedAverageNumberOfDilutedSharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="WeightedAverageNumberOfDilutedSharesOutstanding" xlink:title="definition: StatementLineItems to WeightedAverageNumberOfDilutedSharesOutstanding" order="21.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:label="OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:title="OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:title="definition: StatementLineItems to OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" order="22.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" xlink:label="OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" xlink:title="OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" xlink:title="definition: StatementLineItems to OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" order="23.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="OtherComprehensiveIncomeLossNetOfTax" xlink:title="OtherComprehensiveIncomeLossNetOfTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherComprehensiveIncomeLossNetOfTax" xlink:title="definition: StatementLineItems to OtherComprehensiveIncomeLossNetOfTax" order="24.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToNoncontrollingInterest" xlink:label="OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToNoncontrollingInterest" xlink:title="OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToNoncontrollingInterest" xlink:title="definition: StatementLineItems to OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToNoncontrollingInterest" order="25.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:label="OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:title="OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:title="definition: StatementLineItems to OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" order="26.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="InterestExpenseNonoperating" xlink:title="InterestExpenseNonoperating" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="InterestExpenseNonoperating" xlink:title="definition: StatementLineItems to InterestExpenseNonoperating" order="27.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInterest" xlink:label="InvestmentIncomeInterest" xlink:title="InvestmentIncomeInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="InvestmentIncomeInterest" xlink:title="definition: StatementLineItems to InvestmentIncomeInterest" order="28.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" xlink:label="ForeignCurrencyTransactionGainLossBeforeTax" xlink:title="ForeignCurrencyTransactionGainLossBeforeTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ForeignCurrencyTransactionGainLossBeforeTax" xlink:title="definition: StatementLineItems to ForeignCurrencyTransactionGainLossBeforeTax" order="29.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" xlink:label="EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" xlink:title="EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" xlink:title="definition: StatementLineItems to EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" order="30.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeDividend" xlink:label="InvestmentIncomeDividend" xlink:title="InvestmentIncomeDividend" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="InvestmentIncomeDividend" xlink:title="definition: StatementLineItems to InvestmentIncomeDividend" order="31.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InvestmentIncomeDividendRelatedParty" xlink:label="InvestmentIncomeDividendRelatedParty" xlink:title="InvestmentIncomeDividendRelatedParty" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="InvestmentIncomeDividendRelatedParty" xlink:title="definition: StatementLineItems to InvestmentIncomeDividendRelatedParty" order="32.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiGainLoss" xlink:label="EquitySecuritiesFvNiGainLoss" xlink:title="EquitySecuritiesFvNiGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EquitySecuritiesFvNiGainLoss" xlink:title="definition: StatementLineItems to EquitySecuritiesFvNiGainLoss" order="33.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesTradingGainLoss" xlink:label="DebtSecuritiesTradingGainLoss" xlink:title="DebtSecuritiesTradingGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DebtSecuritiesTradingGainLoss" xlink:title="definition: StatementLineItems to DebtSecuritiesTradingGainLoss" order="34.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="OtherNonoperatingIncomeExpense" xlink:title="OtherNonoperatingIncomeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherNonoperatingIncomeExpense" xlink:title="definition: StatementLineItems to OtherNonoperatingIncomeExpense" order="35.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="NonoperatingIncomeExpense" xlink:title="NonoperatingIncomeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="NonoperatingIncomeExpense" xlink:title="definition: StatementLineItems to NonoperatingIncomeExpense" order="36.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeconsolidationGainOrLossAmount" xlink:label="DeconsolidationGainOrLossAmount" xlink:title="DeconsolidationGainOrLossAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DeconsolidationGainOrLossAmount" xlink:title="definition: StatementLineItems to DeconsolidationGainOrLossAmount" order="37.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:label="EquitySecuritiesFvNiUnrealizedGainLoss" xlink:title="EquitySecuritiesFvNiUnrealizedGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EquitySecuritiesFvNiUnrealizedGainLoss" xlink:title="definition: StatementLineItems to EquitySecuritiesFvNiUnrealizedGainLoss" order="38.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:label="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:title="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:title="definition: StatementLineItems to EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" order="39.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherOperatingIncomeExpenseNet" xlink:label="OtherOperatingIncomeExpenseNet" xlink:title="OtherOperatingIncomeExpenseNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherOperatingIncomeExpenseNet" xlink:title="definition: StatementLineItems to OtherOperatingIncomeExpenseNet" order="40.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VoyageExpenses" xlink:label="VoyageExpenses" xlink:title="VoyageExpenses" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="VoyageExpenses" xlink:title="definition: StatementLineItems to VoyageExpenses" order="41.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingCostsAndExpenses" xlink:label="OperatingCostsAndExpenses" xlink:title="OperatingCostsAndExpenses" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OperatingCostsAndExpenses" xlink:title="definition: StatementLineItems to OperatingCostsAndExpenses" order="42.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:label="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:title="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:title="definition: StatementLineItems to CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" order="43.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionManagementFees" xlink:label="RelatedPartyTransactionManagementFees" xlink:title="RelatedPartyTransactionManagementFees" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="RelatedPartyTransactionManagementFees" xlink:title="definition: StatementLineItems to RelatedPartyTransactionManagementFees" order="44.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="DepreciationDepletionAndAmortization" xlink:title="DepreciationDepletionAndAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DepreciationDepletionAndAmortization" xlink:title="definition: StatementLineItems to DepreciationDepletionAndAmortization" order="45.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:label="ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:title="ImpairmentOfLongLivedAssetsToBeDisposedOf" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:title="definition: StatementLineItems to ImpairmentOfLongLivedAssetsToBeDisposedOf" order="46.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForDoubtfulAccounts" xlink:label="ProvisionForDoubtfulAccounts" xlink:title="ProvisionForDoubtfulAccounts" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ProvisionForDoubtfulAccounts" xlink:title="definition: StatementLineItems to ProvisionForDoubtfulAccounts" order="47.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="GeneralAndAdministrativeExpense" xlink:title="GeneralAndAdministrativeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="GeneralAndAdministrativeExpense" xlink:title="definition: StatementLineItems to GeneralAndAdministrativeExpense" order="48.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnDispositionOfAssets" xlink:label="GainLossOnDispositionOfAssets" xlink:title="GainLossOnDispositionOfAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="GainLossOnDispositionOfAssets" xlink:title="definition: StatementLineItems to GainLossOnDispositionOfAssets" order="49.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostsAndExpenses" xlink:label="CostsAndExpenses" xlink:title="CostsAndExpenses" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CostsAndExpenses" xlink:title="definition: StatementLineItems to CostsAndExpenses" order="50.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLeaseIncome" xlink:label="OperatingLeaseLeaseIncome" xlink:title="OperatingLeaseLeaseIncome" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OperatingLeaseLeaseIncome" xlink:title="definition: StatementLineItems to OperatingLeaseLeaseIncome" order="51.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="definition: StatementLineItems to RevenueFromContractWithCustomerExcludingAssessedTax" order="52.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Revenues" xlink:label="Revenues" xlink:title="Revenues" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="Revenues" xlink:title="definition: StatementLineItems to Revenues" order="53.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="StatementTable" xlink:title="StatementTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="StatementLineItems" xlink:to="StatementTable" xlink:title="definition: StatementLineItems to StatementTable" order="54.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis" xlink:label="ProductOrServiceAxis" xlink:title="ProductOrServiceAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="StatementTable" xlink:to="ProductOrServiceAxis" xlink:title="definition: StatementTable to ProductOrServiceAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="ProductsAndServicesDomain" xlink:title="ProductsAndServicesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ProductOrServiceAxis" xlink:to="ProductsAndServicesDomain" xlink:title="definition: ProductOrServiceAxis to ProductsAndServicesDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="ProductsAndServicesDomain_2" xlink:title="ProductsAndServicesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ProductOrServiceAxis" xlink:to="ProductsAndServicesDomain_2" xlink:title="definition: ProductOrServiceAxis to ProductsAndServicesDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TimeCharterMember" xlink:label="TimeCharterMember" xlink:title="TimeCharterMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ProductsAndServicesDomain" xlink:to="TimeCharterMember" xlink:title="definition: ProductsAndServicesDomain to TimeCharterMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PoolAgreementsMember" xlink:label="PoolAgreementsMember" xlink:title="PoolAgreementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ProductsAndServicesDomain" xlink:to="PoolAgreementsMember" xlink:title="definition: ProductsAndServicesDomain to PoolAgreementsMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/UnauditedInterimCondensedConsolidatedStatementsOfComprehensiveIncomeParenthetical">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="StatementLineItems" xlink:title="StatementLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesAbstract" xlink:label="RevenuesAbstract" xlink:title="RevenuesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="RevenuesAbstract" xlink:title="definition: StatementLineItems to RevenuesAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostsAndExpensesAbstract" xlink:label="CostsAndExpensesAbstract" xlink:title="CostsAndExpensesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CostsAndExpensesAbstract" xlink:title="definition: StatementLineItems to CostsAndExpensesAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherIncomeAndExpensesAbstract" xlink:label="OtherIncomeAndExpensesAbstract" xlink:title="OtherIncomeAndExpensesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherIncomeAndExpensesAbstract" xlink:title="definition: StatementLineItems to OtherIncomeAndExpensesAbstract" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="InterestExpenseNonoperating" xlink:title="InterestExpenseNonoperating" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="InterestExpenseNonoperating" xlink:title="definition: StatementLineItems to InterestExpenseNonoperating" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VoyageExpenses" xlink:label="VoyageExpenses" xlink:title="VoyageExpenses" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="VoyageExpenses" xlink:title="definition: StatementLineItems to VoyageExpenses" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:label="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:title="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:title="definition: StatementLineItems to CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:label="ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:title="ImpairmentOfLongLivedAssetsToBeDisposedOf" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:title="definition: StatementLineItems to ImpairmentOfLongLivedAssetsToBeDisposedOf" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="GeneralAndAdministrativeExpense" xlink:title="GeneralAndAdministrativeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="GeneralAndAdministrativeExpense" xlink:title="definition: StatementLineItems to GeneralAndAdministrativeExpense" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnDispositionOfAssets" xlink:label="GainLossOnDispositionOfAssets" xlink:title="GainLossOnDispositionOfAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="GainLossOnDispositionOfAssets" xlink:title="definition: StatementLineItems to GainLossOnDispositionOfAssets" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="definition: StatementLineItems to RevenueFromContractWithCustomerExcludingAssessedTax" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="StatementTable" xlink:title="StatementTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="StatementLineItems" xlink:to="StatementTable" xlink:title="definition: StatementLineItems to StatementTable" order="11.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="StatementTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: StatementTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/UnauditedCondensedConsolidatedStatementsOfShareholdersEquityAndMezzanineEquity">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="StatementLineItems" xlink:title="StatementLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="IncreaseDecreaseInStockholdersEquityRollForward" xlink:title="IncreaseDecreaseInStockholdersEquityRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="IncreaseDecreaseInStockholdersEquityRollForward" xlink:title="definition: StatementLineItems to IncreaseDecreaseInStockholdersEquityRollForward" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward" xlink:label="IncreaseDecreaseInTemporaryEquityRollForward" xlink:title="IncreaseDecreaseInTemporaryEquityRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="IncreaseDecreaseInTemporaryEquityRollForward" xlink:title="definition: StatementLineItems to IncreaseDecreaseInTemporaryEquityRollForward" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent" xlink:label="TemporaryEquityCarryingAmountAttributableToParent" xlink:title="TemporaryEquityCarryingAmountAttributableToParent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquityCarryingAmountAttributableToParent" xlink:title="definition: StatementLineItems to TemporaryEquityCarryingAmountAttributableToParent" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesOutstanding" xlink:label="TemporaryEquitySharesOutstanding" xlink:title="TemporaryEquitySharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquitySharesOutstanding" xlink:title="definition: StatementLineItems to TemporaryEquitySharesOutstanding" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityAccretionOfDividends" xlink:label="TemporaryEquityAccretionOfDividends" xlink:title="TemporaryEquityAccretionOfDividends" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="TemporaryEquityAccretionOfDividends" xlink:title="definition: StatementLineItems to TemporaryEquityAccretionOfDividends" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:title="StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:title="definition: StatementLineItems to StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesOutstanding" xlink:label="SharesOutstanding" xlink:title="SharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="SharesOutstanding" xlink:title="definition: StatementLineItems to SharesOutstanding" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsPreferredStockCash" xlink:label="DividendsPreferredStockCash" xlink:title="DividendsPreferredStockCash" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="DividendsPreferredStockCash" xlink:title="definition: StatementLineItems to DividendsPreferredStockCash" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseInCarryingAmountOfRedeemablePreferredStock" xlink:label="IncreaseInCarryingAmountOfRedeemablePreferredStock" xlink:title="IncreaseInCarryingAmountOfRedeemablePreferredStock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="IncreaseInCarryingAmountOfRedeemablePreferredStock" xlink:title="definition: StatementLineItems to IncreaseInCarryingAmountOfRedeemablePreferredStock" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" xlink:label="MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" xlink:title="MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" xlink:title="definition: StatementLineItems to MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ChangesInOwnershipOfSubsidiaryWithoutLossOfControl" xlink:label="ChangesInOwnershipOfSubsidiaryWithoutLossOfControl" xlink:title="ChangesInOwnershipOfSubsidiaryWithoutLossOfControl" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ChangesInOwnershipOfSubsidiaryWithoutLossOfControl" xlink:title="definition: StatementLineItems to ChangesInOwnershipOfSubsidiaryWithoutLossOfControl" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NoncontrollingInterestFromFirstTimeConsolidationEffects" xlink:label="NoncontrollingInterestFromFirstTimeConsolidationEffects" xlink:title="NoncontrollingInterestFromFirstTimeConsolidationEffects" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="NoncontrollingInterestFromFirstTimeConsolidationEffects" xlink:title="definition: StatementLineItems to NoncontrollingInterestFromFirstTimeConsolidationEffects" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" xlink:label="IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" xlink:title="IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" xlink:title="definition: StatementLineItems to IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance" xlink:label="NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance" xlink:title="NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance" xlink:title="definition: StatementLineItems to NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="OtherComprehensiveIncomeLossNetOfTax" xlink:title="OtherComprehensiveIncomeLossNetOfTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="OtherComprehensiveIncomeLossNetOfTax" xlink:title="definition: StatementLineItems to OtherComprehensiveIncomeLossNetOfTax" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="ProfitLoss" xlink:title="ProfitLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="StatementLineItems" xlink:to="ProfitLoss" xlink:title="definition: StatementLineItems to ProfitLoss" order="16.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="StatementTable" xlink:title="StatementTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="StatementLineItems" xlink:to="StatementTable" xlink:title="definition: StatementLineItems to StatementTable" order="17.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="StatementEquityComponentsAxis" xlink:title="StatementEquityComponentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="StatementTable" xlink:to="StatementEquityComponentsAxis" xlink:title="definition: StatementTable to StatementEquityComponentsAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="EquityComponentDomain" xlink:title="EquityComponentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementEquityComponentsAxis" xlink:to="EquityComponentDomain" xlink:title="definition: StatementEquityComponentsAxis to EquityComponentDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="EquityComponentDomain_2" xlink:title="EquityComponentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementEquityComponentsAxis" xlink:to="EquityComponentDomain_2" xlink:title="definition: StatementEquityComponentsAxis to EquityComponentDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="CommonStockMember" xlink:title="CommonStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityComponentDomain" xlink:to="CommonStockMember" xlink:title="definition: EquityComponentDomain to CommonStockMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember" xlink:label="PreferredStockMember" xlink:title="PreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityComponentDomain" xlink:to="PreferredStockMember" xlink:title="definition: EquityComponentDomain to PreferredStockMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ParValueOfSharesIssuedMember" xlink:label="ParValueOfSharesIssuedMember" xlink:title="ParValueOfSharesIssuedMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityComponentDomain" xlink:to="ParValueOfSharesIssuedMember" xlink:title="definition: EquityComponentDomain to ParValueOfSharesIssuedMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="AdditionalPaidInCapitalMember" xlink:title="AdditionalPaidInCapitalMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityComponentDomain" xlink:to="AdditionalPaidInCapitalMember" xlink:title="definition: EquityComponentDomain to AdditionalPaidInCapitalMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember" xlink:label="RetainedEarningsMember" xlink:title="RetainedEarningsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityComponentDomain" xlink:to="RetainedEarningsMember" xlink:title="definition: EquityComponentDomain to RetainedEarningsMember" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="AccumulatedOtherComprehensiveIncomeMember" xlink:title="AccumulatedOtherComprehensiveIncomeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityComponentDomain" xlink:to="AccumulatedOtherComprehensiveIncomeMember" xlink:title="definition: EquityComponentDomain to AccumulatedOtherComprehensiveIncomeMember" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ParentMember" xlink:label="ParentMember" xlink:title="ParentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityComponentDomain" xlink:to="ParentMember" xlink:title="definition: EquityComponentDomain to ParentMember" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncontrollingInterestMember" xlink:label="NoncontrollingInterestMember" xlink:title="NoncontrollingInterestMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityComponentDomain" xlink:to="NoncontrollingInterestMember" xlink:title="definition: EquityComponentDomain to NoncontrollingInterestMember" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="StatementClassOfStockAxis" xlink:title="StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="StatementTable" xlink:to="StatementClassOfStockAxis" xlink:title="definition: StatementTable to StatementClassOfStockAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain_2" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain_2" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember" xlink:label="SeriesBPreferredStockMember" xlink:title="SeriesBPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesBPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesBPreferredStockMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesDPreferredStockMember" xlink:label="SeriesDPreferredStockMember" xlink:title="SeriesDPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesDPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesDPreferredStockMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/UnauditedInterimCondensedConsolidatedStatementsOfCashFlows" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/BasisOfPresentationAndGeneralInformation" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SignificantAccountingPoliciesAndRecentAccountingPronouncements" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedParties" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/DeferredChargesNet" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/FairValueOfAcquiredTimeCharters" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/VesselsNet">
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureLineItems" xlink:label="PropertyPlantAndEquipmentDisclosureLineItems" xlink:title="PropertyPlantAndEquipmentDisclosureLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:label="PropertyPlantAndEquipmentDisclosureTextBlock" xlink:title="PropertyPlantAndEquipmentDisclosureTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentDisclosureLineItems" xlink:to="PropertyPlantAndEquipmentDisclosureTextBlock" xlink:title="definition: PropertyPlantAndEquipmentDisclosureLineItems to PropertyPlantAndEquipmentDisclosureTextBlock" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureTable" xlink:label="PropertyPlantAndEquipmentDisclosureTable" xlink:title="PropertyPlantAndEquipmentDisclosureTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentDisclosureLineItems" xlink:to="PropertyPlantAndEquipmentDisclosureTable" xlink:title="definition: PropertyPlantAndEquipmentDisclosureLineItems to PropertyPlantAndEquipmentDisclosureTable" order="2.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="PropertyPlantAndEquipmentDisclosureTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: PropertyPlantAndEquipmentDisclosureTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VesselsMember" xlink:label="VesselsMember" xlink:title="VesselsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="VesselsMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to VesselsMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/PropertyPlantAndEquipmentNet">
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureLineItems" xlink:label="PropertyPlantAndEquipmentDisclosureLineItems" xlink:title="PropertyPlantAndEquipmentDisclosureLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:label="PropertyPlantAndEquipmentDisclosureTextBlock" xlink:title="PropertyPlantAndEquipmentDisclosureTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentDisclosureLineItems" xlink:to="PropertyPlantAndEquipmentDisclosureTextBlock" xlink:title="definition: PropertyPlantAndEquipmentDisclosureLineItems to PropertyPlantAndEquipmentDisclosureTextBlock" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureTable" xlink:label="PropertyPlantAndEquipmentDisclosureTable" xlink:title="PropertyPlantAndEquipmentDisclosureTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentDisclosureLineItems" xlink:to="PropertyPlantAndEquipmentDisclosureTable" xlink:title="definition: PropertyPlantAndEquipmentDisclosureLineItems to PropertyPlantAndEquipmentDisclosureTable" order="2.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="PropertyPlantAndEquipmentDisclosureTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: PropertyPlantAndEquipmentDisclosureTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyAndEquipmentMember" xlink:label="PropertyAndEquipmentMember" xlink:title="PropertyAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="PropertyAndEquipmentMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to PropertyAndEquipmentMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IntangibleAssetsNet" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/Goodwill" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityMethodInvestments" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LongtermDebt" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InvestmentInEquitySecuritiesEquityInvestments">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems" xlink:label="ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems" xlink:title="ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:label="InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:title="InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems" xlink:to="InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:title="definition: ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems to InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsTable" xlink:label="ScheduleOfTradingSecuritiesAndOtherTradingAssetsTable" xlink:title="ScheduleOfTradingSecuritiesAndOtherTradingAssetsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems" xlink:to="ScheduleOfTradingSecuritiesAndOtherTradingAssetsTable" xlink:title="definition: ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems to ScheduleOfTradingSecuritiesAndOtherTradingAssetsTable" order="2.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="FairValueByAssetClassAxis" xlink:title="FairValueByAssetClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfTradingSecuritiesAndOtherTradingAssetsTable" xlink:to="FairValueByAssetClassAxis" xlink:title="definition: ScheduleOfTradingSecuritiesAndOtherTradingAssetsTable to FairValueByAssetClassAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:title="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByAssetClassAxis" xlink:to="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:title="definition: FairValueByAssetClassAxis to FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" xlink:title="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByAssetClassAxis" xlink:to="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" xlink:title="definition: FairValueByAssetClassAxis to FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesMember" xlink:label="EquitySecuritiesMember" xlink:title="EquitySecuritiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:to="EquitySecuritiesMember" xlink:title="definition: FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain to EquitySecuritiesMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InvestmentInDebtSecurities">
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InvestmentsInDebtAndEquitySecuritiesDisclosureLineItems" xlink:label="InvestmentsInDebtAndEquitySecuritiesDisclosureLineItems" xlink:title="InvestmentsInDebtAndEquitySecuritiesDisclosureLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:label="InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:title="InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentsInDebtAndEquitySecuritiesDisclosureLineItems" xlink:to="InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:title="definition: InvestmentsInDebtAndEquitySecuritiesDisclosureLineItems to InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InvestmentsInDebtAndEquitySecuritiesDisclosureTable" xlink:label="InvestmentsInDebtAndEquitySecuritiesDisclosureTable" xlink:title="InvestmentsInDebtAndEquitySecuritiesDisclosureTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="InvestmentsInDebtAndEquitySecuritiesDisclosureLineItems" xlink:to="InvestmentsInDebtAndEquitySecuritiesDisclosureTable" xlink:title="definition: InvestmentsInDebtAndEquitySecuritiesDisclosureLineItems to InvestmentsInDebtAndEquitySecuritiesDisclosureTable" order="2.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="FairValueByAssetClassAxis" xlink:title="FairValueByAssetClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="InvestmentsInDebtAndEquitySecuritiesDisclosureTable" xlink:to="FairValueByAssetClassAxis" xlink:title="definition: InvestmentsInDebtAndEquitySecuritiesDisclosureTable to FairValueByAssetClassAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:title="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByAssetClassAxis" xlink:to="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:title="definition: FairValueByAssetClassAxis to FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" xlink:title="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByAssetClassAxis" xlink:to="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" xlink:title="definition: FairValueByAssetClassAxis to FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesMember" xlink:label="DebtSecuritiesMember" xlink:title="DebtSecuritiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:to="DebtSecuritiesMember" xlink:title="definition: FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain to DebtSecuritiesMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityCapitalStructure" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosures" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/Leases" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/CommitmentsAndContingencies" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EarningsPerCommonShare" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/Revenues" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/VesselOperatingExpensesAndVoyageExpenses" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/GeneralAndAdministrativeExpenses" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/CostOfRevenueFromServices" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InterestAndFinanceCosts" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IncomeTaxes" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SharebasedCompensation" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SegmentInformation" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SubsequentEvents" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/BasisOfPresentationAndGeneralInformationPolicies" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SignificantAccountingPoliciesAndRecentAccountingPronouncementsPolicies" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedPartiesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/DeferredChargesNetTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/VesselsNetTables">
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureLineItems" xlink:label="PropertyPlantAndEquipmentDisclosureLineItems" xlink:title="PropertyPlantAndEquipmentDisclosureLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:label="PropertyPlantAndEquipmentTextBlock" xlink:title="PropertyPlantAndEquipmentTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentDisclosureLineItems" xlink:to="PropertyPlantAndEquipmentTextBlock" xlink:title="definition: PropertyPlantAndEquipmentDisclosureLineItems to PropertyPlantAndEquipmentTextBlock" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureTable" xlink:label="PropertyPlantAndEquipmentDisclosureTable" xlink:title="PropertyPlantAndEquipmentDisclosureTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentDisclosureLineItems" xlink:to="PropertyPlantAndEquipmentDisclosureTable" xlink:title="definition: PropertyPlantAndEquipmentDisclosureLineItems to PropertyPlantAndEquipmentDisclosureTable" order="2.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="PropertyPlantAndEquipmentDisclosureTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: PropertyPlantAndEquipmentDisclosureTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VesselsMember" xlink:label="VesselsMember" xlink:title="VesselsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="VesselsMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to VesselsMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/PropertyPlantAndEquipmentNetTables">
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureLineItems" xlink:label="PropertyPlantAndEquipmentDisclosureLineItems" xlink:title="PropertyPlantAndEquipmentDisclosureLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:label="PropertyPlantAndEquipmentTextBlock" xlink:title="PropertyPlantAndEquipmentTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentDisclosureLineItems" xlink:to="PropertyPlantAndEquipmentTextBlock" xlink:title="definition: PropertyPlantAndEquipmentDisclosureLineItems to PropertyPlantAndEquipmentTextBlock" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureTable" xlink:label="PropertyPlantAndEquipmentDisclosureTable" xlink:title="PropertyPlantAndEquipmentDisclosureTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentDisclosureLineItems" xlink:to="PropertyPlantAndEquipmentDisclosureTable" xlink:title="definition: PropertyPlantAndEquipmentDisclosureLineItems to PropertyPlantAndEquipmentDisclosureTable" order="2.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="PropertyPlantAndEquipmentDisclosureTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: PropertyPlantAndEquipmentDisclosureTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyAndEquipmentMember" xlink:label="PropertyAndEquipmentMember" xlink:title="PropertyAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="PropertyAndEquipmentMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to PropertyAndEquipmentMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IntangibleAssetsNetTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/GoodwillTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityMethodInvestmentsTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LongtermDebtTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InvestmentInEquitySecuritiesEquityInvestmentsTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InvestmentInDebtSecuritiesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosuresTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LeasesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/CommitmentsAndContingenciesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EarningsPerCommonShareTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/RevenuesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/VesselOperatingExpensesAndVoyageExpensesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/GeneralAndAdministrativeExpensesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/CostOfRevenueFromServicesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InterestAndFinanceCostsTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IncomeTaxesTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SharebasedCompensationTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SegmentInformationTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/BasisOfPresentationAndGeneralInformationGeneralInformationDetails">
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_OrganizationLineItems" xlink:label="OrganizationLineItems" xlink:title="OrganizationLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_BasisOfPresentationAndOrganizationAbstract" xlink:label="BasisOfPresentationAndOrganizationAbstract" xlink:title="BasisOfPresentationAndOrganizationAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OrganizationLineItems" xlink:to="BasisOfPresentationAndOrganizationAbstract" xlink:title="definition: OrganizationLineItems to BasisOfPresentationAndOrganizationAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="CommonStockParOrStatedValuePerShare" xlink:title="CommonStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OrganizationLineItems" xlink:to="CommonStockParOrStatedValuePerShare" xlink:title="definition: OrganizationLineItems to CommonStockParOrStatedValuePerShare" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PercentageOfSharesHeld" xlink:label="PercentageOfSharesHeld" xlink:title="PercentageOfSharesHeld" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OrganizationLineItems" xlink:to="PercentageOfSharesHeld" xlink:title="definition: OrganizationLineItems to PercentageOfSharesHeld" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfVessels" xlink:label="NumberOfVessels" xlink:title="NumberOfVessels" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OrganizationLineItems" xlink:to="NumberOfVessels" xlink:title="definition: OrganizationLineItems to NumberOfVessels" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VesselDeadWeightTonnage" xlink:label="VesselDeadWeightTonnage" xlink:title="VesselDeadWeightTonnage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OrganizationLineItems" xlink:to="VesselDeadWeightTonnage" xlink:title="definition: OrganizationLineItems to VesselDeadWeightTonnage" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfWhollyOwnedSubsidiaries" xlink:label="NumberOfWhollyOwnedSubsidiaries" xlink:title="NumberOfWhollyOwnedSubsidiaries" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OrganizationLineItems" xlink:to="NumberOfWhollyOwnedSubsidiaries" xlink:title="definition: OrganizationLineItems to NumberOfWhollyOwnedSubsidiaries" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfMajorityOwnedSubsidiaries" xlink:label="NumberOfMajorityOwnedSubsidiaries" xlink:title="NumberOfMajorityOwnedSubsidiaries" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OrganizationLineItems" xlink:to="NumberOfMajorityOwnedSubsidiaries" xlink:title="definition: OrganizationLineItems to NumberOfMajorityOwnedSubsidiaries" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfMajorityOwnedSubsidiariesDissolved" xlink:label="NumberOfMajorityOwnedSubsidiariesDissolved" xlink:title="NumberOfMajorityOwnedSubsidiariesDissolved" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OrganizationLineItems" xlink:to="NumberOfMajorityOwnedSubsidiariesDissolved" xlink:title="definition: OrganizationLineItems to NumberOfMajorityOwnedSubsidiariesDissolved" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_OrganizationTable" xlink:label="OrganizationTable" xlink:title="OrganizationTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="OrganizationLineItems" xlink:to="OrganizationTable" xlink:title="definition: OrganizationLineItems to OrganizationTable" order="9.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="OrganizationTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: OrganizationTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="CounterpartyNameAxis" xlink:title="CounterpartyNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="OrganizationTable" xlink:to="CounterpartyNameAxis" xlink:title="definition: OrganizationTable to CounterpartyNameAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ThalassaInvestmentCoSAMember" xlink:label="ThalassaInvestmentCoSAMember" xlink:title="ThalassaInvestmentCoSAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="ThalassaInvestmentCoSAMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to ThalassaInvestmentCoSAMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="StatementClassOfStockAxis" xlink:title="StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="OrganizationTable" xlink:to="StatementClassOfStockAxis" xlink:title="definition: OrganizationTable to StatementClassOfStockAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain_2" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain_2" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember" xlink:label="SeriesBPreferredStockMember" xlink:title="SeriesBPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesBPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesBPreferredStockMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="OrganizationTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: OrganizationTable to PropertyPlantAndEquipmentByTypeAxis" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_KamsarmaxVesselMember" xlink:label="KamsarmaxVesselMember" xlink:title="KamsarmaxVesselMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="KamsarmaxVesselMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to KamsarmaxVesselMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PanamaxVesselMember" xlink:label="PanamaxVesselMember" xlink:title="PanamaxVesselMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="PanamaxVesselMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to PanamaxVesselMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_UltramaxDryBulkVesselMember" xlink:label="UltramaxDryBulkVesselMember" xlink:title="UltramaxDryBulkVesselMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="UltramaxDryBulkVesselMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to UltramaxDryBulkVesselMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TEUContainership1850Member" xlink:label="TEUContainership1850Member" xlink:title="TEUContainership1850Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="TEUContainership1850Member" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to TEUContainership1850Member" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="StatementGeographicalAxis" xlink:title="StatementGeographicalAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="OrganizationTable" xlink:to="StatementGeographicalAxis" xlink:title="definition: OrganizationTable to StatementGeographicalAxis" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="SegmentGeographicalDomain" xlink:title="SegmentGeographicalDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementGeographicalAxis" xlink:to="SegmentGeographicalDomain" xlink:title="definition: StatementGeographicalAxis to SegmentGeographicalDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="SegmentGeographicalDomain_2" xlink:title="SegmentGeographicalDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementGeographicalAxis" xlink:to="SegmentGeographicalDomain_2" xlink:title="definition: StatementGeographicalAxis to SegmentGeographicalDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_MH" xlink:label="MH" xlink:title="MH" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="MH" xlink:title="definition: SegmentGeographicalDomain to MH" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CY" xlink:label="CY" xlink:title="CY" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="CY" xlink:title="definition: SegmentGeographicalDomain to CY" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_DE" xlink:label="DE" xlink:title="DE" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="DE" xlink:title="definition: SegmentGeographicalDomain to DE" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_NL" xlink:label="NL" xlink:title="NL" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="NL" xlink:title="definition: SegmentGeographicalDomain to NL" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_SingaporeColombiaPanamaAndThePeopleSRepublicOfChinaMember" xlink:label="SingaporeColombiaPanamaAndThePeopleSRepublicOfChinaMember" xlink:title="SingaporeColombiaPanamaAndThePeopleSRepublicOfChinaMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="SingaporeColombiaPanamaAndThePeopleSRepublicOfChinaMember" xlink:title="definition: SegmentGeographicalDomain to SingaporeColombiaPanamaAndThePeopleSRepublicOfChinaMember" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_SG" xlink:label="SG" xlink:title="SG" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="SG" xlink:title="definition: SegmentGeographicalDomain to SG" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CO" xlink:label="CO" xlink:title="CO" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="CO" xlink:title="definition: SegmentGeographicalDomain to CO" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_PA" xlink:label="PA" xlink:title="PA" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="PA" xlink:title="definition: SegmentGeographicalDomain to PA" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CN" xlink:label="CN" xlink:title="CN" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="CN" xlink:title="definition: SegmentGeographicalDomain to CN" order="9.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedPartiesSummaryDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="RelatedPartyTransactionLineItems" xlink:title="RelatedPartyTransactionLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionDueFromToRelatedPartiesAbstract" xlink:label="RelatedPartyTransactionDueFromToRelatedPartiesAbstract" xlink:title="RelatedPartyTransactionDueFromToRelatedPartiesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionDueFromToRelatedPartiesAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionDueFromToRelatedPartiesAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="AssetsAbstract" xlink:title="AssetsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="AssetsAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to AssetsAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAbstract" xlink:label="LiabilitiesAbstract" xlink:title="LiabilitiesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="LiabilitiesAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to LiabilitiesAbstract" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesCurrent" xlink:label="OtherLiabilitiesCurrent" xlink:title="OtherLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherLiabilitiesCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherLiabilitiesCurrent" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivablesNetCurrent" xlink:label="OtherReceivablesNetCurrent" xlink:title="OtherReceivablesNetCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherReceivablesNetCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherReceivablesNetCurrent" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:label="OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:title="OtherReceivableAfterAllowanceForCreditLossNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherReceivableAfterAllowanceForCreditLossNoncurrent" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InvestmentInRelatedParty" xlink:label="InvestmentInRelatedParty" xlink:title="InvestmentInRelatedParty" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="InvestmentInRelatedParty" xlink:title="definition: RelatedPartyTransactionLineItems to InvestmentInRelatedParty" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="RelatedPartyTransactionLineItems" xlink:to="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="definition: RelatedPartyTransactionLineItems to ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="8.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="CounterpartyNameAxis" xlink:title="CounterpartyNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="CounterpartyNameAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to CounterpartyNameAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_CastorShipsSAMember" xlink:label="CastorShipsSAMember" xlink:title="CastorShipsSAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="CastorShipsSAMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to CastorShipsSAMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ToroCorpMember" xlink:label="ToroCorpMember" xlink:title="ToroCorpMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="ToroCorpMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to ToroCorpMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCMunchmeyerPetersenCapitalAGMember" xlink:label="MPCMunchmeyerPetersenCapitalAGMember" xlink:title="MPCMunchmeyerPetersenCapitalAGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="MPCMunchmeyerPetersenCapitalAGMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to MPCMunchmeyerPetersenCapitalAGMember" order="3.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedPartiesCastorShipsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="RelatedPartyTransactionLineItems" xlink:title="RelatedPartyTransactionLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:label="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionManagementFees" xlink:label="RelatedPartyTransactionManagementFees" xlink:title="RelatedPartyTransactionManagementFees" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionManagementFees" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionManagementFees" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionCommissions" xlink:label="RelatedPartyTransactionCommissions" xlink:title="RelatedPartyTransactionCommissions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionCommissions" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionCommissions" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionSaleAndPurchaseCommissions" xlink:label="RelatedPartyTransactionSaleAndPurchaseCommissions" xlink:title="RelatedPartyTransactionSaleAndPurchaseCommissions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionSaleAndPurchaseCommissions" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionSaleAndPurchaseCommissions" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfVesselsSold" xlink:label="NumberOfVesselsSold" xlink:title="NumberOfVesselsSold" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="NumberOfVesselsSold" xlink:title="definition: RelatedPartyTransactionLineItems to NumberOfVesselsSold" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionFlatManagementFees" xlink:label="RelatedPartyTransactionFlatManagementFees" xlink:title="RelatedPartyTransactionFlatManagementFees" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionFlatManagementFees" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionFlatManagementFees" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FundingPeriodForAdvanceToCoverVesselDailyOperatingCosts" xlink:label="FundingPeriodForAdvanceToCoverVesselDailyOperatingCosts" xlink:title="FundingPeriodForAdvanceToCoverVesselDailyOperatingCosts" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="FundingPeriodForAdvanceToCoverVesselDailyOperatingCosts" xlink:title="definition: RelatedPartyTransactionLineItems to FundingPeriodForAdvanceToCoverVesselDailyOperatingCosts" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:label="OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:title="OtherReceivableAfterAllowanceForCreditLossNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherReceivableAfterAllowanceForCreditLossNoncurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherReceivableAfterAllowanceForCreditLossNoncurrent" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivablesNetCurrent" xlink:label="OtherReceivablesNetCurrent" xlink:title="OtherReceivablesNetCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherReceivablesNetCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherReceivablesNetCurrent" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesCurrent" xlink:label="OtherLiabilitiesCurrent" xlink:title="OtherLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherLiabilitiesCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherLiabilitiesCurrent" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="RelatedPartyTransactionLineItems" xlink:to="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="definition: RelatedPartyTransactionLineItems to ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="11.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="CounterpartyNameAxis" xlink:title="CounterpartyNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="CounterpartyNameAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to CounterpartyNameAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_CastorShipsSAMember" xlink:label="CastorShipsSAMember" xlink:title="CastorShipsSAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="CastorShipsSAMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to CastorShipsSAMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="RelatedPartyTransactionAxis" xlink:title="RelatedPartyTransactionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="RelatedPartyTransactionDomain" xlink:title="RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionAxis" xlink:to="RelatedPartyTransactionDomain" xlink:title="definition: RelatedPartyTransactionAxis to RelatedPartyTransactionDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="RelatedPartyTransactionDomain_2" xlink:title="RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionAxis" xlink:to="RelatedPartyTransactionDomain_2" xlink:title="definition: RelatedPartyTransactionAxis to RelatedPartyTransactionDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_CastorShipManagementAgreementsMember" xlink:label="CastorShipManagementAgreementsMember" xlink:title="CastorShipManagementAgreementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionDomain" xlink:to="CastorShipManagementAgreementsMember" xlink:title="definition: RelatedPartyTransactionDomain to CastorShipManagementAgreementsMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_AmendedCastorShipManagementAgreementsMember" xlink:label="AmendedCastorShipManagementAgreementsMember" xlink:title="AmendedCastorShipManagementAgreementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionDomain" xlink:to="AmendedCastorShipManagementAgreementsMember" xlink:title="definition: RelatedPartyTransactionDomain to AmendedCastorShipManagementAgreementsMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_WorkingCapitalGuaranteeAdvancesMember" xlink:label="WorkingCapitalGuaranteeAdvancesMember" xlink:title="WorkingCapitalGuaranteeAdvancesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionDomain" xlink:to="WorkingCapitalGuaranteeAdvancesMember" xlink:title="definition: RelatedPartyTransactionDomain to WorkingCapitalGuaranteeAdvancesMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_OperatingExpensesPaymentsMember" xlink:label="OperatingExpensesPaymentsMember" xlink:title="OperatingExpensesPaymentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionDomain" xlink:to="OperatingExpensesPaymentsMember" xlink:title="definition: RelatedPartyTransactionDomain to OperatingExpensesPaymentsMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_HarperPetersenHamburgMember" xlink:label="HarperPetersenHamburgMember" xlink:title="HarperPetersenHamburgMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionDomain" xlink:to="HarperPetersenHamburgMember" xlink:title="definition: RelatedPartyTransactionDomain to HarperPetersenHamburgMember" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="IncomeStatementLocationAxis" xlink:title="IncomeStatementLocationAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="IncomeStatementLocationAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to IncomeStatementLocationAxis" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="IncomeStatementLocationDomain" xlink:title="IncomeStatementLocationDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="IncomeStatementLocationAxis" xlink:to="IncomeStatementLocationDomain" xlink:title="definition: IncomeStatementLocationAxis to IncomeStatementLocationDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="IncomeStatementLocationDomain_2" xlink:title="IncomeStatementLocationDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="IncomeStatementLocationAxis" xlink:to="IncomeStatementLocationDomain_2" xlink:title="definition: IncomeStatementLocationAxis to IncomeStatementLocationDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_WindFarmProductionCapacity" xlink:label="WindFarmProductionCapacity" xlink:title="WindFarmProductionCapacity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeStatementLocationDomain" xlink:to="WindFarmProductionCapacity" xlink:title="definition: IncomeStatementLocationDomain to WindFarmProductionCapacity" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="GeneralAndAdministrativeExpenseMember" xlink:title="GeneralAndAdministrativeExpenseMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeStatementLocationDomain" xlink:to="GeneralAndAdministrativeExpenseMember" xlink:title="definition: IncomeStatementLocationDomain to GeneralAndAdministrativeExpenseMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseMember" xlink:label="InterestExpenseMember" xlink:title="InterestExpenseMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeStatementLocationDomain" xlink:to="InterestExpenseMember" xlink:title="definition: IncomeStatementLocationDomain to InterestExpenseMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DeferredLoanFeesMember" xlink:label="DeferredLoanFeesMember" xlink:title="DeferredLoanFeesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeStatementLocationDomain" xlink:to="DeferredLoanFeesMember" xlink:title="definition: IncomeStatementLocationDomain to DeferredLoanFeesMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to PropertyPlantAndEquipmentByTypeAxis" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VesselsMember" xlink:label="VesselsMember" xlink:title="VesselsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="VesselsMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to VesselsMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PanamaxVesselMember" xlink:label="PanamaxVesselMember" xlink:title="PanamaxVesselMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="VesselsMember" xlink:to="PanamaxVesselMember" xlink:title="definition: VesselsMember to PanamaxVesselMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ContainerVesselMember" xlink:label="ContainerVesselMember" xlink:title="ContainerVesselMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="VesselsMember" xlink:to="ContainerVesselMember" xlink:title="definition: VesselsMember to ContainerVesselMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedPartiesPavimarDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="RelatedPartyTransactionLineItems" xlink:title="RelatedPartyTransactionLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:label="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionFixedDailyFee" xlink:label="RelatedPartyTransactionFixedDailyFee" xlink:title="RelatedPartyTransactionFixedDailyFee" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionFixedDailyFee" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionFixedDailyFee" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionManagementFees" xlink:label="RelatedPartyTransactionManagementFees" xlink:title="RelatedPartyTransactionManagementFees" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionManagementFees" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionManagementFees" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivablesNetCurrent" xlink:label="OtherReceivablesNetCurrent" xlink:title="OtherReceivablesNetCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherReceivablesNetCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherReceivablesNetCurrent" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesCurrent" xlink:label="OtherLiabilitiesCurrent" xlink:title="OtherLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherLiabilitiesCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherLiabilitiesCurrent" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="RelatedPartyTransactionLineItems" xlink:to="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="definition: RelatedPartyTransactionLineItems to ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="CounterpartyNameAxis" xlink:title="CounterpartyNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="CounterpartyNameAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to CounterpartyNameAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PavimarSAMember" xlink:label="PavimarSAMember" xlink:title="PavimarSAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="PavimarSAMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to PavimarSAMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="RelatedPartyTransactionAxis" xlink:title="RelatedPartyTransactionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="RelatedPartyTransactionDomain" xlink:title="RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionAxis" xlink:to="RelatedPartyTransactionDomain" xlink:title="definition: RelatedPartyTransactionAxis to RelatedPartyTransactionDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="RelatedPartyTransactionDomain_2" xlink:title="RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionAxis" xlink:to="RelatedPartyTransactionDomain_2" xlink:title="definition: RelatedPartyTransactionAxis to RelatedPartyTransactionDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TechnicalShipmanagementAgreementsMember" xlink:label="TechnicalShipmanagementAgreementsMember" xlink:title="TechnicalShipmanagementAgreementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionDomain" xlink:to="TechnicalShipmanagementAgreementsMember" xlink:title="definition: RelatedPartyTransactionDomain to TechnicalShipmanagementAgreementsMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedPartiesInvestmentInRelatedPartyDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="RelatedPartyTransactionLineItems" xlink:title="RelatedPartyTransactionLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InvestmentInRelatedPartyAbstract" xlink:label="InvestmentInRelatedPartyAbstract" xlink:title="InvestmentInRelatedPartyAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="InvestmentInRelatedPartyAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to InvestmentInRelatedPartyAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="PreferredStockSharesIssued" xlink:title="PreferredStockSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PreferredStockSharesIssued" xlink:title="definition: RelatedPartyTransactionLineItems to PreferredStockSharesIssued" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PreferredStockStatedValuePerShare" xlink:label="PreferredStockStatedValuePerShare" xlink:title="PreferredStockStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PreferredStockStatedValuePerShare" xlink:title="definition: RelatedPartyTransactionLineItems to PreferredStockStatedValuePerShare" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="PreferredStockParOrStatedValuePerShare" xlink:title="PreferredStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PreferredStockParOrStatedValuePerShare" xlink:title="definition: RelatedPartyTransactionLineItems to PreferredStockParOrStatedValuePerShare" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PreferredStockVotingRightsNumberOfVotes" xlink:label="PreferredStockVotingRightsNumberOfVotes" xlink:title="PreferredStockVotingRightsNumberOfVotes" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PreferredStockVotingRightsNumberOfVotes" xlink:title="definition: RelatedPartyTransactionLineItems to PreferredStockVotingRightsNumberOfVotes" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PercentageOfVolumeWeightedAveragePrice" xlink:label="PercentageOfVolumeWeightedAveragePrice" xlink:title="PercentageOfVolumeWeightedAveragePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PercentageOfVolumeWeightedAveragePrice" xlink:title="definition: RelatedPartyTransactionLineItems to PercentageOfVolumeWeightedAveragePrice" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfConsecutiveTradingDayCommencingOnDistributionDate" xlink:label="NumberOfConsecutiveTradingDayCommencingOnDistributionDate" xlink:title="NumberOfConsecutiveTradingDayCommencingOnDistributionDate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="NumberOfConsecutiveTradingDayCommencingOnDistributionDate" xlink:title="definition: RelatedPartyTransactionLineItems to NumberOfConsecutiveTradingDayCommencingOnDistributionDate" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfConsecutiveTradingDayExpiringOnTradingDay" xlink:label="NumberOfConsecutiveTradingDayExpiringOnTradingDay" xlink:title="NumberOfConsecutiveTradingDayExpiringOnTradingDay" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="NumberOfConsecutiveTradingDayExpiringOnTradingDay" xlink:title="definition: RelatedPartyTransactionLineItems to NumberOfConsecutiveTradingDayExpiringOnTradingDay" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockConvertibleConversionPrice" xlink:label="PreferredStockConvertibleConversionPrice" xlink:title="PreferredStockConvertibleConversionPrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PreferredStockConvertibleConversionPrice" xlink:title="definition: RelatedPartyTransactionLineItems to PreferredStockConvertibleConversionPrice" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InvestmentInRelatedParty" xlink:label="InvestmentInRelatedParty" xlink:title="InvestmentInRelatedParty" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="InvestmentInRelatedParty" xlink:title="definition: RelatedPartyTransactionLineItems to InvestmentInRelatedParty" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsPayableCurrent" xlink:label="DividendsPayableCurrent" xlink:title="DividendsPayableCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DividendsPayableCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to DividendsPayableCurrent" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="PreferredStockDividendRatePercentage" xlink:title="PreferredStockDividendRatePercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PreferredStockDividendRatePercentage" xlink:title="definition: RelatedPartyTransactionLineItems to PreferredStockDividendRatePercentage" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DividendMultiplierFactor" xlink:label="DividendMultiplierFactor" xlink:title="DividendMultiplierFactor" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DividendMultiplierFactor" xlink:title="definition: RelatedPartyTransactionLineItems to DividendMultiplierFactor" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InvestmentIncomeDividendRelatedParty" xlink:label="InvestmentIncomeDividendRelatedParty" xlink:title="InvestmentIncomeDividendRelatedParty" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="InvestmentIncomeDividendRelatedParty" xlink:title="definition: RelatedPartyTransactionLineItems to InvestmentIncomeDividendRelatedParty" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="RelatedPartyTransactionLineItems" xlink:to="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="definition: RelatedPartyTransactionLineItems to ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="15.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="StatementClassOfStockAxis" xlink:title="StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="StatementClassOfStockAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to StatementClassOfStockAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain_2" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain_2" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="SeriesAPreferredStockMember" xlink:title="SeriesAPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesAPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesAPreferredStockMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionsByRelatedPartyAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="CounterpartyNameAxis" xlink:title="CounterpartyNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="CounterpartyNameAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to CounterpartyNameAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ToroCorpMember" xlink:label="ToroCorpMember" xlink:title="ToroCorpMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="ToroCorpMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to ToroCorpMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="RangeAxis" xlink:title="RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RangeAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RangeAxis" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="RangeMember" xlink:title="RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RangeAxis" xlink:to="RangeMember" xlink:title="definition: RangeAxis to RangeMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="RangeMember_2" xlink:title="RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RangeAxis" xlink:to="RangeMember_2" xlink:title="definition: RangeAxis to RangeMember_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="MinimumMember" xlink:title="MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RangeMember" xlink:to="MinimumMember" xlink:title="definition: RangeMember to MinimumMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="MaximumMember" xlink:title="MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RangeMember" xlink:to="MaximumMember" xlink:title="definition: RangeMember to MaximumMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedPartiesIssuanceOfSeriesDPreferredSharesToToroDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="RelatedPartyTransactionLineItems" xlink:title="RelatedPartyTransactionLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:label="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="PreferredStockSharesIssued" xlink:title="PreferredStockSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PreferredStockSharesIssued" xlink:title="definition: RelatedPartyTransactionLineItems to PreferredStockSharesIssued" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="PreferredStockDividendRatePercentage" xlink:title="PreferredStockDividendRatePercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="PreferredStockDividendRatePercentage" xlink:title="definition: RelatedPartyTransactionLineItems to PreferredStockDividendRatePercentage" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:label="ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:title="ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:title="definition: RelatedPartyTransactionLineItems to ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsPayableCurrent" xlink:label="DividendsPayableCurrent" xlink:title="DividendsPayableCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DividendsPayableCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to DividendsPayableCurrent" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesCurrent" xlink:label="OtherLiabilitiesCurrent" xlink:title="OtherLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherLiabilitiesCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherLiabilitiesCurrent" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="RelatedPartyTransactionLineItems" xlink:to="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="definition: RelatedPartyTransactionLineItems to ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="7.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="CounterpartyNameAxis" xlink:title="CounterpartyNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="CounterpartyNameAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to CounterpartyNameAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ToroCorpMember" xlink:label="ToroCorpMember" xlink:title="ToroCorpMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="ToroCorpMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to ToroCorpMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="StatementClassOfStockAxis" xlink:title="StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="StatementClassOfStockAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to StatementClassOfStockAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain_2" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain_2" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesDPreferredStockMember" xlink:label="SeriesDPreferredStockMember" xlink:title="SeriesDPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesDPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesDPreferredStockMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedPartiesLongtermDebtRelatedPartyDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="RelatedPartyTransactionLineItems" xlink:title="RelatedPartyTransactionLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:label="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfSecuredDebt" xlink:label="RepaymentsOfSecuredDebt" xlink:title="RepaymentsOfSecuredDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RepaymentsOfSecuredDebt" xlink:title="definition: RelatedPartyTransactionLineItems to RepaymentsOfSecuredDebt" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtWeightedAverageInterestRateOverTime" xlink:label="LongTermDebtWeightedAverageInterestRateOverTime" xlink:title="LongTermDebtWeightedAverageInterestRateOverTime" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="LongTermDebtWeightedAverageInterestRateOverTime" xlink:title="definition: RelatedPartyTransactionLineItems to LongTermDebtWeightedAverageInterestRateOverTime" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseLongTermDebt" xlink:label="InterestExpenseLongTermDebt" xlink:title="InterestExpenseLongTermDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="InterestExpenseLongTermDebt" xlink:title="definition: RelatedPartyTransactionLineItems to InterestExpenseLongTermDebt" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="DebtInstrumentFaceAmount" xlink:title="DebtInstrumentFaceAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DebtInstrumentFaceAmount" xlink:title="definition: RelatedPartyTransactionLineItems to DebtInstrumentFaceAmount" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTerm" xlink:label="DebtInstrumentTerm" xlink:title="DebtInstrumentTerm" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DebtInstrumentTerm" xlink:title="definition: RelatedPartyTransactionLineItems to DebtInstrumentTerm" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration" xlink:label="DebtInstrumentVariableInterestRateTypeExtensibleEnumeration" xlink:title="DebtInstrumentVariableInterestRateTypeExtensibleEnumeration" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DebtInstrumentVariableInterestRateTypeExtensibleEnumeration" xlink:title="definition: RelatedPartyTransactionLineItems to DebtInstrumentVariableInterestRateTypeExtensibleEnumeration" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="DebtInstrumentBasisSpreadOnVariableRate1" xlink:title="DebtInstrumentBasisSpreadOnVariableRate1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DebtInstrumentBasisSpreadOnVariableRate1" xlink:title="definition: RelatedPartyTransactionLineItems to DebtInstrumentBasisSpreadOnVariableRate1" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfShipOwningSubsidiariesEnteringIntoTermLoanFacility" xlink:label="NumberOfShipOwningSubsidiariesEnteringIntoTermLoanFacility" xlink:title="NumberOfShipOwningSubsidiariesEnteringIntoTermLoanFacility" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="NumberOfShipOwningSubsidiariesEnteringIntoTermLoanFacility" xlink:title="definition: RelatedPartyTransactionLineItems to NumberOfShipOwningSubsidiariesEnteringIntoTermLoanFacility" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DebtInstrumentNumberOfConsecutivePaymentInstallments" xlink:label="DebtInstrumentNumberOfConsecutivePaymentInstallments" xlink:title="DebtInstrumentNumberOfConsecutivePaymentInstallments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DebtInstrumentNumberOfConsecutivePaymentInstallments" xlink:title="definition: RelatedPartyTransactionLineItems to DebtInstrumentNumberOfConsecutivePaymentInstallments" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFrequencyOfPeriodicPayment" xlink:label="DebtInstrumentFrequencyOfPeriodicPayment" xlink:title="DebtInstrumentFrequencyOfPeriodicPayment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DebtInstrumentFrequencyOfPeriodicPayment" xlink:title="definition: RelatedPartyTransactionLineItems to DebtInstrumentFrequencyOfPeriodicPayment" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentPeriodicPayment" xlink:label="DebtInstrumentPeriodicPayment" xlink:title="DebtInstrumentPeriodicPayment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DebtInstrumentPeriodicPayment" xlink:title="definition: RelatedPartyTransactionLineItems to DebtInstrumentPeriodicPayment" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" xlink:label="DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" xlink:title="DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" xlink:title="definition: RelatedPartyTransactionLineItems to DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfVesselsMortgagedAsCollateral" xlink:label="NumberOfVesselsMortgagedAsCollateral" xlink:title="NumberOfVesselsMortgagedAsCollateral" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="NumberOfVesselsMortgagedAsCollateral" xlink:title="definition: RelatedPartyTransactionLineItems to NumberOfVesselsMortgagedAsCollateral" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="RelatedPartyTransactionLineItems" xlink:to="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="definition: RelatedPartyTransactionLineItems to ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="15.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="CounterpartyNameAxis" xlink:title="CounterpartyNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="CounterpartyNameAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to CounterpartyNameAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ToroCorpMember" xlink:label="ToroCorpMember" xlink:title="ToroCorpMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="ToroCorpMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to ToroCorpMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="DebtInstrumentAxis" xlink:title="DebtInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="DebtInstrumentAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to DebtInstrumentAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain_2" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain_2" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_SeniorTermLoanFacilityMember" xlink:label="SeniorTermLoanFacilityMember" xlink:title="SeniorTermLoanFacilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentNameDomain" xlink:to="SeniorTermLoanFacilityMember" xlink:title="definition: DebtInstrumentNameDomain to SeniorTermLoanFacilityMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/TransactionsWithRelatedPartiesMpcCapitalRelatedPartiesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="RelatedPartyTransactionLineItems" xlink:title="RelatedPartyTransactionLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RevenuesFromServicesWithRelatedPartiesAbstract" xlink:label="RevenuesFromServicesWithRelatedPartiesAbstract" xlink:title="RevenuesFromServicesWithRelatedPartiesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RevenuesFromServicesWithRelatedPartiesAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to RevenuesFromServicesWithRelatedPartiesAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:label="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" xlink:title="definition: RelatedPartyTransactionLineItems to RelatedPartyTransactionDueFromToRelatedPartiesCurrentAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="EquityMethodInvestmentOwnershipPercentage" xlink:title="EquityMethodInvestmentOwnershipPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="EquityMethodInvestmentOwnershipPercentage" xlink:title="definition: RelatedPartyTransactionLineItems to EquityMethodInvestmentOwnershipPercentage" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsOwnershipPercentageAcquiredDuringPeriod" xlink:label="EquityMethodInvestmentsOwnershipPercentageAcquiredDuringPeriod" xlink:title="EquityMethodInvestmentsOwnershipPercentageAcquiredDuringPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="EquityMethodInvestmentsOwnershipPercentageAcquiredDuringPeriod" xlink:title="definition: RelatedPartyTransactionLineItems to EquityMethodInvestmentsOwnershipPercentageAcquiredDuringPeriod" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherReceivablesNetCurrent" xlink:label="OtherReceivablesNetCurrent" xlink:title="OtherReceivablesNetCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="OtherReceivablesNetCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to OtherReceivablesNetCurrent" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="AccountsReceivableNetCurrent" xlink:title="AccountsReceivableNetCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="AccountsReceivableNetCurrent" xlink:title="definition: RelatedPartyTransactionLineItems to AccountsReceivableNetCurrent" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyTransactionLineItems" xlink:to="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="definition: RelatedPartyTransactionLineItems to RevenueFromContractWithCustomerExcludingAssessedTax" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="RelatedPartyTransactionLineItems" xlink:to="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:title="definition: RelatedPartyTransactionLineItems to ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="8.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="BusinessAcquisitionAxis" xlink:title="BusinessAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="BusinessAcquisitionAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to BusinessAcquisitionAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="BusinessAcquisitionAcquireeDomain" xlink:title="BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="BusinessAcquisitionAxis" xlink:to="BusinessAcquisitionAcquireeDomain" xlink:title="definition: BusinessAcquisitionAxis to BusinessAcquisitionAcquireeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="BusinessAcquisitionAcquireeDomain_2" xlink:title="BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="BusinessAcquisitionAxis" xlink:to="BusinessAcquisitionAcquireeDomain_2" xlink:title="definition: BusinessAcquisitionAxis to BusinessAcquisitionAcquireeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCCCSILTDMember" xlink:label="MPCCCSILTDMember" xlink:title="MPCCCSILTDMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="BusinessAcquisitionAcquireeDomain" xlink:to="MPCCCSILTDMember" xlink:title="definition: BusinessAcquisitionAcquireeDomain to MPCCCSILTDMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="definition: ScheduleOfRelatedPartyTransactionsByRelatedPartyTable to ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain_2" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain_2" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain_2" order="2.0" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCCCSILTDMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCCCSILTDMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCContainerShipsASAMember" xlink:label="MPCContainerShipsASAMember" xlink:title="MPCContainerShipsASAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCContainerShipsASAMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCContainerShipsASAMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_WilhelmsenAhrenkielShipManagementGmbHCoKGMember" xlink:label="WilhelmsenAhrenkielShipManagementGmbHCoKGMember" xlink:title="WilhelmsenAhrenkielShipManagementGmbHCoKGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="WilhelmsenAhrenkielShipManagementGmbHCoKGMember" xlink:title="definition: EquityMethodInvesteeNameDomain to WilhelmsenAhrenkielShipManagementGmbHCoKGMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCEnergySolutionsNVMember" xlink:label="MPCEnergySolutionsNVMember" xlink:title="MPCEnergySolutionsNVMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCEnergySolutionsNVMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCEnergySolutionsNVMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCCaribbeanCleanEnergyLimitedBarbadosMember" xlink:label="MPCCaribbeanCleanEnergyLimitedBarbadosMember" xlink:title="MPCCaribbeanCleanEnergyLimitedBarbadosMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCCaribbeanCleanEnergyLimitedBarbadosMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCCaribbeanCleanEnergyLimitedBarbadosMember" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_OtherEquityMethodInvestmentMember" xlink:label="OtherEquityMethodInvestmentMember" xlink:title="OtherEquityMethodInvestmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="OtherEquityMethodInvestmentMember" xlink:title="definition: EquityMethodInvesteeNameDomain to OtherEquityMethodInvestmentMember" order="6.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/DeferredChargesNetDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostLineItems" xlink:label="CapitalizedContractCostLineItems" xlink:title="CapitalizedContractCostLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostNetClassifiedAbstract" xlink:label="CapitalizedContractCostNetClassifiedAbstract" xlink:title="CapitalizedContractCostNetClassifiedAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="CapitalizedContractCostLineItems" xlink:to="CapitalizedContractCostNetClassifiedAbstract" xlink:title="definition: CapitalizedContractCostLineItems to CapitalizedContractCostNetClassifiedAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfVesselsCompletingScheduledDrydockingRepairs" xlink:label="NumberOfVesselsCompletingScheduledDrydockingRepairs" xlink:title="NumberOfVesselsCompletingScheduledDrydockingRepairs" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="CapitalizedContractCostLineItems" xlink:to="NumberOfVesselsCompletingScheduledDrydockingRepairs" xlink:title="definition: CapitalizedContractCostLineItems to NumberOfVesselsCompletingScheduledDrydockingRepairs" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostTable" xlink:label="CapitalizedContractCostTable" xlink:title="CapitalizedContractCostTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="CapitalizedContractCostLineItems" xlink:to="CapitalizedContractCostTable" xlink:title="definition: CapitalizedContractCostLineItems to CapitalizedContractCostTable" order="3.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="CapitalizedContractCostTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: CapitalizedContractCostTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DryBulkCarrierMember" xlink:label="DryBulkCarrierMember" xlink:title="DryBulkCarrierMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="DryBulkCarrierMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to DryBulkCarrierMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/FairValueOfAcquiredTimeChartersDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="PropertyPlantAndEquipmentLineItems" xlink:title="PropertyPlantAndEquipmentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BelowMarketLeaseAbstract" xlink:label="BelowMarketLeaseAbstract" xlink:title="BelowMarketLeaseAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="BelowMarketLeaseAbstract" xlink:title="definition: PropertyPlantAndEquipmentLineItems to BelowMarketLeaseAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FiniteLivedIntangibleAssetOffMarketLeaseFavorableNet" xlink:label="FiniteLivedIntangibleAssetOffMarketLeaseFavorableNet" xlink:title="FiniteLivedIntangibleAssetOffMarketLeaseFavorableNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="FiniteLivedIntangibleAssetOffMarketLeaseFavorableNet" xlink:title="definition: PropertyPlantAndEquipmentLineItems to FiniteLivedIntangibleAssetOffMarketLeaseFavorableNet" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:label="FiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:title="FiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="FiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:title="definition: PropertyPlantAndEquipmentLineItems to FiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:label="UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:title="UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" xlink:title="definition: PropertyPlantAndEquipmentLineItems to UnamortizedFiniteLivedIntangibleLiabilityOffMarketLeaseUnfavorableNet" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfAboveAndBelowMarketLeases" xlink:label="AmortizationOfAboveAndBelowMarketLeases" xlink:title="AmortizationOfAboveAndBelowMarketLeases" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="AmortizationOfAboveAndBelowMarketLeases" xlink:title="definition: PropertyPlantAndEquipmentLineItems to AmortizationOfAboveAndBelowMarketLeases" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="definition: PropertyPlantAndEquipmentLineItems to ScheduleOfPropertyPlantAndEquipmentTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: ScheduleOfPropertyPlantAndEquipmentTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVRaphaelaMember" xlink:label="MVRaphaelaMember" xlink:title="MVRaphaelaMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="MVRaphaelaMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to MVRaphaelaMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVMagicSaturnMember" xlink:label="MVMagicSaturnMember" xlink:title="MVMagicSaturnMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="MVMagicSaturnMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to MVMagicSaturnMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/VesselsNetNetBookValueDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="PropertyPlantAndEquipmentLineItems" xlink:title="PropertyPlantAndEquipmentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MovementInPropertyPlantAndEquipmentRollForward" xlink:label="MovementInPropertyPlantAndEquipmentRollForward" xlink:title="MovementInPropertyPlantAndEquipmentRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="MovementInPropertyPlantAndEquipmentRollForward" xlink:title="definition: PropertyPlantAndEquipmentLineItems to MovementInPropertyPlantAndEquipmentRollForward" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MovementInAccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipmentRollForward" xlink:label="MovementInAccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipmentRollForward" xlink:title="MovementInAccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipmentRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="MovementInAccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipmentRollForward" xlink:title="definition: PropertyPlantAndEquipmentLineItems to MovementInAccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipmentRollForward" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract" xlink:label="PropertyPlantAndEquipmentNetByTypeAbstract" xlink:title="PropertyPlantAndEquipmentNetByTypeAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentNetByTypeAbstract" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentNetByTypeAbstract" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="PropertyPlantAndEquipmentNet" xlink:title="PropertyPlantAndEquipmentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentNet" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentNet" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:title="AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:title="definition: PropertyPlantAndEquipmentLineItems to AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Depreciation" xlink:label="Depreciation" xlink:title="Depreciation" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="Depreciation" xlink:title="definition: PropertyPlantAndEquipmentLineItems to Depreciation" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="PropertyPlantAndEquipmentGross" xlink:title="PropertyPlantAndEquipmentGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentGross" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentGross" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAdditions" xlink:label="PropertyPlantAndEquipmentAdditions" xlink:title="PropertyPlantAndEquipmentAdditions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentAdditions" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentAdditions" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="definition: PropertyPlantAndEquipmentLineItems to ScheduleOfPropertyPlantAndEquipmentTable" order="9.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: ScheduleOfPropertyPlantAndEquipmentTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VesselsMember" xlink:label="VesselsMember" xlink:title="VesselsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="VesselsMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to VesselsMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/VesselsNetVesselAcquisitionsAndOtherCapitalExpendituresDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="PropertyPlantAndEquipmentLineItems" xlink:title="PropertyPlantAndEquipmentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNetAbstract" xlink:label="PropertyPlantAndEquipmentNetAbstract" xlink:title="PropertyPlantAndEquipmentNetAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentNetAbstract" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentNetAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAdditions" xlink:label="PropertyPlantAndEquipmentAdditions" xlink:title="PropertyPlantAndEquipmentAdditions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentAdditions" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentAdditions" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfImpairedVessels" xlink:label="NumberOfImpairedVessels" xlink:title="NumberOfImpairedVessels" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="NumberOfImpairedVessels" xlink:title="definition: PropertyPlantAndEquipmentLineItems to NumberOfImpairedVessels" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="definition: PropertyPlantAndEquipmentLineItems to ScheduleOfPropertyPlantAndEquipmentTable" order="4.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: ScheduleOfPropertyPlantAndEquipmentTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVMagicJupiterMember" xlink:label="MVMagicJupiterMember" xlink:title="MVMagicJupiterMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="MVMagicJupiterMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to MVMagicJupiterMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVMagicSaturnMember" xlink:label="MVMagicSaturnMember" xlink:title="MVMagicSaturnMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="MVMagicSaturnMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to MVMagicSaturnMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InstallationOfNewEquipmentPursuantToEnvironmentalRegulationsMember" xlink:label="InstallationOfNewEquipmentPursuantToEnvironmentalRegulationsMember" xlink:title="InstallationOfNewEquipmentPursuantToEnvironmentalRegulationsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="InstallationOfNewEquipmentPursuantToEnvironmentalRegulationsMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to InstallationOfNewEquipmentPursuantToEnvironmentalRegulationsMember" order="3.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/PropertyPlantAndEquipmentNetPropertyAndEquipmentByMajorAssetClassesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="PropertyPlantAndEquipmentLineItems" xlink:title="PropertyPlantAndEquipmentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract" xlink:label="PropertyPlantAndEquipmentNetByTypeAbstract" xlink:title="PropertyPlantAndEquipmentNetByTypeAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentNetByTypeAbstract" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentNetByTypeAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="PropertyPlantAndEquipmentNet" xlink:title="PropertyPlantAndEquipmentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentNet" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentNet" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_WindFarmProductionCapacity" xlink:label="WindFarmProductionCapacity" xlink:title="WindFarmProductionCapacity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="WindFarmProductionCapacity" xlink:title="definition: PropertyPlantAndEquipmentLineItems to WindFarmProductionCapacity" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfWindTurbineGenerators" xlink:label="NumberOfWindTurbineGenerators" xlink:title="NumberOfWindTurbineGenerators" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="NumberOfWindTurbineGenerators" xlink:title="definition: PropertyPlantAndEquipmentLineItems to NumberOfWindTurbineGenerators" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationNonproduction" xlink:label="DepreciationNonproduction" xlink:title="DepreciationNonproduction" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="DepreciationNonproduction" xlink:title="definition: PropertyPlantAndEquipmentLineItems to DepreciationNonproduction" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="definition: PropertyPlantAndEquipmentLineItems to ScheduleOfPropertyPlantAndEquipmentTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: ScheduleOfPropertyPlantAndEquipmentTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyAndEquipmentMember" xlink:label="PropertyAndEquipmentMember" xlink:title="PropertyAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="PropertyAndEquipmentMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to PropertyAndEquipmentMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_WindTurbinesMember" xlink:label="WindTurbinesMember" xlink:title="WindTurbinesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyAndEquipmentMember" xlink:to="WindTurbinesMember" xlink:title="definition: PropertyAndEquipmentMember to WindTurbinesMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="LeaseholdImprovementsMember" xlink:title="LeaseholdImprovementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyAndEquipmentMember" xlink:to="LeaseholdImprovementsMember" xlink:title="definition: PropertyAndEquipmentMember to LeaseholdImprovementsMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="FurnitureAndFixturesMember" xlink:title="FurnitureAndFixturesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyAndEquipmentMember" xlink:to="FurnitureAndFixturesMember" xlink:title="definition: PropertyAndEquipmentMember to FurnitureAndFixturesMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OfficeEquipmentMember" xlink:label="OfficeEquipmentMember" xlink:title="OfficeEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyAndEquipmentMember" xlink:to="OfficeEquipmentMember" xlink:title="definition: PropertyAndEquipmentMember to OfficeEquipmentMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:label="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:title="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:title="definition: ScheduleOfPropertyPlantAndEquipmentTable to IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:label="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:title="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:to="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:title="definition: IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis to DisposalGroupsIncludingDiscontinuedOperationsNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:label="DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2" xlink:title="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:to="DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2" xlink:title="definition: IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis to DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" xlink:label="EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" xlink:title="EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:to="EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" xlink:title="definition: DisposalGroupsIncludingDiscontinuedOperationsNameDomain to EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/PropertyPlantAndEquipmentNetGrossCarryingAmountAndAccumulatedDepreciationDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="PropertyPlantAndEquipmentLineItems" xlink:title="PropertyPlantAndEquipmentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract" xlink:label="PropertyPlantAndEquipmentNetByTypeAbstract" xlink:title="PropertyPlantAndEquipmentNetByTypeAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentNetByTypeAbstract" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentNetByTypeAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="PropertyPlantAndEquipmentGross" xlink:title="PropertyPlantAndEquipmentGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentGross" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentGross" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:title="AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:title="definition: PropertyPlantAndEquipmentLineItems to AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="PropertyPlantAndEquipmentNet" xlink:title="PropertyPlantAndEquipmentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentNet" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentNet" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyPlantAndEquipmentDisclosureTable" xlink:label="PropertyPlantAndEquipmentDisclosureTable" xlink:title="PropertyPlantAndEquipmentDisclosureTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="PropertyPlantAndEquipmentLineItems" xlink:to="PropertyPlantAndEquipmentDisclosureTable" xlink:title="definition: PropertyPlantAndEquipmentLineItems to PropertyPlantAndEquipmentDisclosureTable" order="5.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="PropertyPlantAndEquipmentDisclosureTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: PropertyPlantAndEquipmentDisclosureTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PropertyAndEquipmentMember" xlink:label="PropertyAndEquipmentMember" xlink:title="PropertyAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="PropertyAndEquipmentMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to PropertyAndEquipmentMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_WindTurbinesMember" xlink:label="WindTurbinesMember" xlink:title="WindTurbinesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyAndEquipmentMember" xlink:to="WindTurbinesMember" xlink:title="definition: PropertyAndEquipmentMember to WindTurbinesMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="LeaseholdImprovementsMember" xlink:title="LeaseholdImprovementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyAndEquipmentMember" xlink:to="LeaseholdImprovementsMember" xlink:title="definition: PropertyAndEquipmentMember to LeaseholdImprovementsMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="FurnitureAndFixturesMember" xlink:title="FurnitureAndFixturesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyAndEquipmentMember" xlink:to="FurnitureAndFixturesMember" xlink:title="definition: PropertyAndEquipmentMember to FurnitureAndFixturesMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OfficeEquipmentMember" xlink:label="OfficeEquipmentMember" xlink:title="OfficeEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyAndEquipmentMember" xlink:to="OfficeEquipmentMember" xlink:title="definition: PropertyAndEquipmentMember to OfficeEquipmentMember" order="4.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IntangibleAssetsNetDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="FiniteLivedIntangibleAssetsLineItems" xlink:title="FiniteLivedIntangibleAssetsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNetAbstract" xlink:label="FiniteLivedIntangibleAssetsNetAbstract" xlink:title="FiniteLivedIntangibleAssetsNetAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsNetAbstract" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsNetAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="FiniteLivedIntangibleAssetsGross" xlink:title="FiniteLivedIntangibleAssetsGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsGross" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsGross" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:title="FiniteLivedIntangibleAssetsAccumulatedAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsAccumulatedAmortization" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="FiniteLivedIntangibleAssetsNet" xlink:title="FiniteLivedIntangibleAssetsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsNet" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsNet" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="AmortizationOfIntangibleAssets" xlink:title="AmortizationOfIntangibleAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="AmortizationOfIntangibleAssets" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to AmortizationOfIntangibleAssets" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsForeignCurrencyTranslationGainLoss" xlink:label="FiniteLivedIntangibleAssetsForeignCurrencyTranslationGainLoss" xlink:title="FiniteLivedIntangibleAssetsForeignCurrencyTranslationGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsForeignCurrencyTranslationGainLoss" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsForeignCurrencyTranslationGainLoss" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:label="FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:title="FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:label="FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:title="FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:label="FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:title="FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:label="FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:title="FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" xlink:label="FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" xlink:title="FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" xlink:label="FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" xlink:title="FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:title="ScheduleOfFiniteLivedIntangibleAssetsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="FiniteLivedIntangibleAssetsLineItems" xlink:to="ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:title="definition: FiniteLivedIntangibleAssetsLineItems to ScheduleOfFiniteLivedIntangibleAssetsTable" order="13.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:title="FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:title="definition: ScheduleOfFiniteLivedIntangibleAssetsTable to FiniteLivedIntangibleAssetsByMajorClassAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:title="FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:title="definition: FiniteLivedIntangibleAssetsByMajorClassAxis to FiniteLivedIntangibleAssetsMajorClassNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="FiniteLivedIntangibleAssetsMajorClassNameDomain_2" xlink:title="FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="FiniteLivedIntangibleAssetsMajorClassNameDomain_2" xlink:title="definition: FiniteLivedIntangibleAssetsByMajorClassAxis to FiniteLivedIntangibleAssetsMajorClassNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TrademarksAndTradeNamesMember" xlink:label="TrademarksAndTradeNamesMember" xlink:title="TrademarksAndTradeNamesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="TrademarksAndTradeNamesMember" xlink:title="definition: FiniteLivedIntangibleAssetsMajorClassNameDomain to TrademarksAndTradeNamesMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="CustomerRelationshipsMember" xlink:title="CustomerRelationshipsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="CustomerRelationshipsMember" xlink:title="definition: FiniteLivedIntangibleAssetsMajorClassNameDomain to CustomerRelationshipsMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OrderOrProductionBacklogMember" xlink:label="OrderOrProductionBacklogMember" xlink:title="OrderOrProductionBacklogMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="OrderOrProductionBacklogMember" xlink:title="definition: FiniteLivedIntangibleAssetsMajorClassNameDomain to OrderOrProductionBacklogMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FavorableContractMember" xlink:label="FavorableContractMember" xlink:title="FavorableContractMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="FavorableContractMember" xlink:title="definition: FiniteLivedIntangibleAssetsMajorClassNameDomain to FavorableContractMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ConcessionAssetsMember" xlink:label="ConcessionAssetsMember" xlink:title="ConcessionAssetsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="ConcessionAssetsMember" xlink:title="definition: FiniteLivedIntangibleAssetsMajorClassNameDomain to ConcessionAssetsMember" order="5.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/GoodwillDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityMethodInvestmentsOwnershipInterestsAndCarryingAmountDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:label="ScheduleOfEquityMethodInvestmentsLineItems" xlink:title="ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:label="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:title="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="EquityMethodInvestmentOwnershipPercentage" xlink:title="EquityMethodInvestmentOwnershipPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentOwnershipPercentage" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentOwnershipPercentage" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments" xlink:label="EquityMethodInvestments" xlink:title="EquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestments" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestments" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentsFairValueDisclosure" xlink:label="EquityMethodInvestmentsFairValueDisclosure" xlink:title="EquityMethodInvestmentsFairValueDisclosure" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentsFairValueDisclosure" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentsFairValueDisclosure" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:label="ScheduleOfEquityMethodInvestmentsTable" xlink:title="ScheduleOfEquityMethodInvestmentsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="ScheduleOfEquityMethodInvestmentsTable" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to ScheduleOfEquityMethodInvestmentsTable" order="5.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfEquityMethodInvestmentsTable" xlink:to="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="definition: ScheduleOfEquityMethodInvestmentsTable to ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain_2" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain_2" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_WilhelmsenAhrenkielShipManagementGmbHCoKGMember" xlink:label="WilhelmsenAhrenkielShipManagementGmbHCoKGMember" xlink:title="WilhelmsenAhrenkielShipManagementGmbHCoKGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="WilhelmsenAhrenkielShipManagementGmbHCoKGMember" xlink:title="definition: EquityMethodInvesteeNameDomain to WilhelmsenAhrenkielShipManagementGmbHCoKGMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_BBAmstelBVMember" xlink:label="BBAmstelBVMember" xlink:title="BBAmstelBVMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="BBAmstelBVMember" xlink:title="definition: EquityMethodInvesteeNameDomain to BBAmstelBVMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCCaribbeanCleanEnergyLimitedBarbadosMember" xlink:label="MPCCaribbeanCleanEnergyLimitedBarbadosMember" xlink:title="MPCCaribbeanCleanEnergyLimitedBarbadosMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCCaribbeanCleanEnergyLimitedBarbadosMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCCaribbeanCleanEnergyLimitedBarbadosMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_BarberShipManagementGermanyGmbHCoKGMember" xlink:label="BarberShipManagementGermanyGmbHCoKGMember" xlink:title="BarberShipManagementGermanyGmbHCoKGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="BarberShipManagementGermanyGmbHCoKGMember" xlink:title="definition: EquityMethodInvesteeNameDomain to BarberShipManagementGermanyGmbHCoKGMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_BestShipGmbHCieKGMember" xlink:label="BestShipGmbHCieKGMember" xlink:title="BestShipGmbHCieKGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="BestShipGmbHCieKGMember" xlink:title="definition: EquityMethodInvesteeNameDomain to BestShipGmbHCieKGMember" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RioJulBeteiligungsGmbHCoKGHamburgMember" xlink:label="RioJulBeteiligungsGmbHCoKGHamburgMember" xlink:title="RioJulBeteiligungsGmbHCoKGHamburgMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="RioJulBeteiligungsGmbHCoKGHamburgMember" xlink:title="definition: EquityMethodInvesteeNameDomain to RioJulBeteiligungsGmbHCoKGHamburgMember" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCStormMaritimeOpportunitiesGmbHCoKGMember" xlink:label="MPCStormMaritimeOpportunitiesGmbHCoKGMember" xlink:title="MPCStormMaritimeOpportunitiesGmbHCoKGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCStormMaritimeOpportunitiesGmbHCoKGMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCStormMaritimeOpportunitiesGmbHCoKGMember" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_OtherEquityMethodInvestmentMember" xlink:label="OtherEquityMethodInvestmentMember" xlink:title="OtherEquityMethodInvestmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="OtherEquityMethodInvestmentMember" xlink:title="definition: EquityMethodInvesteeNameDomain to OtherEquityMethodInvestmentMember" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCContainerShipsASAMember" xlink:label="MPCContainerShipsASAMember" xlink:title="MPCContainerShipsASAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCContainerShipsASAMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCContainerShipsASAMember" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCEnergySolutionsNVMember" xlink:label="MPCEnergySolutionsNVMember" xlink:title="MPCEnergySolutionsNVMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCEnergySolutionsNVMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCEnergySolutionsNVMember" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="RangeAxis" xlink:title="RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfEquityMethodInvestmentsTable" xlink:to="RangeAxis" xlink:title="definition: ScheduleOfEquityMethodInvestmentsTable to RangeAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="RangeMember" xlink:title="RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RangeAxis" xlink:to="RangeMember" xlink:title="definition: RangeAxis to RangeMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="RangeMember_2" xlink:title="RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RangeAxis" xlink:to="RangeMember_2" xlink:title="definition: RangeAxis to RangeMember_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="MinimumMember" xlink:title="MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RangeMember" xlink:to="MinimumMember" xlink:title="definition: RangeMember to MinimumMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="MaximumMember" xlink:title="MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RangeMember" xlink:to="MaximumMember" xlink:title="definition: RangeMember to MaximumMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityMethodInvestmentsEquityMethodInvestmentsDevelopedDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:label="ScheduleOfEquityMethodInvestmentsLineItems" xlink:title="ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:label="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:title="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments" xlink:label="EquityMethodInvestments" xlink:title="EquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestments" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestments" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments" xlink:label="PaymentsToAcquireEquityMethodInvestments" xlink:title="PaymentsToAcquireEquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="PaymentsToAcquireEquityMethodInvestments" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to PaymentsToAcquireEquityMethodInvestments" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:label="EquitySecuritiesFvNiUnrealizedGainLoss" xlink:title="EquitySecuritiesFvNiUnrealizedGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesFvNiUnrealizedGainLoss" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesFvNiUnrealizedGainLoss" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsDistributions" xlink:label="EquityMethodInvestmentsDistributions" xlink:title="EquityMethodInvestmentsDistributions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentsDistributions" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentsDistributions" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsTransfersOther" xlink:label="EquityMethodInvestmentsTransfersOther" xlink:title="EquityMethodInvestmentsTransfersOther" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentsTransfersOther" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentsTransfersOther" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsNetExchangeDifferences" xlink:label="EquityMethodInvestmentsNetExchangeDifferences" xlink:title="EquityMethodInvestmentsNetExchangeDifferences" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentsNetExchangeDifferences" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentsNetExchangeDifferences" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:label="ScheduleOfEquityMethodInvestmentsTable" xlink:title="ScheduleOfEquityMethodInvestmentsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="ScheduleOfEquityMethodInvestmentsTable" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to ScheduleOfEquityMethodInvestmentsTable" order="8.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfEquityMethodInvestmentsTable" xlink:to="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="definition: ScheduleOfEquityMethodInvestmentsTable to ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain_2" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain_2" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RioJulBeteiligungsGmbHCoKGHamburgMember" xlink:label="RioJulBeteiligungsGmbHCoKGHamburgMember" xlink:title="RioJulBeteiligungsGmbHCoKGHamburgMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="RioJulBeteiligungsGmbHCoKGHamburgMember" xlink:title="definition: EquityMethodInvesteeNameDomain to RioJulBeteiligungsGmbHCoKGHamburgMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RioKobeBeteiligungsgesellschaftmbHCoKGHamburgMember" xlink:label="RioKobeBeteiligungsgesellschaftmbHCoKGHamburgMember" xlink:title="RioKobeBeteiligungsgesellschaftmbHCoKGHamburgMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="RioKobeBeteiligungsgesellschaftmbHCoKGHamburgMember" xlink:title="definition: EquityMethodInvesteeNameDomain to RioKobeBeteiligungsgesellschaftmbHCoKGHamburgMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TopekaMPCMaritimeASOsloNorwayMember" xlink:label="TopekaMPCMaritimeASOsloNorwayMember" xlink:title="TopekaMPCMaritimeASOsloNorwayMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="TopekaMPCMaritimeASOsloNorwayMember" xlink:title="definition: EquityMethodInvesteeNameDomain to TopekaMPCMaritimeASOsloNorwayMember" order="3.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityMethodInvestmentsEquityMethodInvestmentsMeasuredAtFairValueDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:label="ScheduleOfEquityMethodInvestmentsLineItems" xlink:title="ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:label="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:title="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentOwnershipInterestAndCarryingAmountAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentOwnershipPercentageHeldByInvestee" xlink:label="EquityMethodInvestmentOwnershipPercentageHeldByInvestee" xlink:title="EquityMethodInvestmentOwnershipPercentageHeldByInvestee" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentOwnershipPercentageHeldByInvestee" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentOwnershipPercentageHeldByInvestee" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PercentageOfSubsidiaryInvestmentReflectedAfterConsolidation" xlink:label="PercentageOfSubsidiaryInvestmentReflectedAfterConsolidation" xlink:title="PercentageOfSubsidiaryInvestmentReflectedAfterConsolidation" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="PercentageOfSubsidiaryInvestmentReflectedAfterConsolidation" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to PercentageOfSubsidiaryInvestmentReflectedAfterConsolidation" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="EquityMethodInvestmentOwnershipPercentage" xlink:title="EquityMethodInvestmentOwnershipPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentOwnershipPercentage" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentOwnershipPercentage" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedLoss" xlink:label="EquitySecuritiesFvNiUnrealizedLoss" xlink:title="EquitySecuritiesFvNiUnrealizedLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesFvNiUnrealizedLoss" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesFvNiUnrealizedLoss" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGain" xlink:label="EquitySecuritiesFvNiUnrealizedGain" xlink:title="EquitySecuritiesFvNiUnrealizedGain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesFvNiUnrealizedGain" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesFvNiUnrealizedGain" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentDividendsOrDistributions" xlink:label="EquityMethodInvestmentDividendsOrDistributions" xlink:title="EquityMethodInvestmentDividendsOrDistributions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentDividendsOrDistributions" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentDividendsOrDistributions" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentAdditionalOwnershipPercentageAcquired" xlink:label="EquityMethodInvestmentAdditionalOwnershipPercentageAcquired" xlink:title="EquityMethodInvestmentAdditionalOwnershipPercentageAcquired" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentAdditionalOwnershipPercentageAcquired" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentAdditionalOwnershipPercentageAcquired" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments" xlink:label="PaymentsToAcquireEquityMethodInvestments" xlink:title="PaymentsToAcquireEquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="PaymentsToAcquireEquityMethodInvestments" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to PaymentsToAcquireEquityMethodInvestments" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:label="ScheduleOfEquityMethodInvestmentsTable" xlink:title="ScheduleOfEquityMethodInvestmentsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="ScheduleOfEquityMethodInvestmentsTable" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to ScheduleOfEquityMethodInvestmentsTable" order="10.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfEquityMethodInvestmentsTable" xlink:to="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="definition: ScheduleOfEquityMethodInvestmentsTable to ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain_2" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain_2" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCCSIGmbHMember" xlink:label="MPCCSIGmbHMember" xlink:title="MPCCSIGmbHMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCCSIGmbHMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCCSIGmbHMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCCCSILTDMember" xlink:label="MPCCCSILTDMember" xlink:title="MPCCCSILTDMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="MPCCSIGmbHMember" xlink:to="MPCCCSILTDMember" xlink:title="definition: MPCCSIGmbHMember to MPCCCSILTDMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCContainerShipsASAMember" xlink:label="MPCContainerShipsASAMember" xlink:title="MPCContainerShipsASAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="MPCCSIGmbHMember" xlink:to="MPCContainerShipsASAMember" xlink:title="definition: MPCCSIGmbHMember to MPCContainerShipsASAMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCEnergySolutionsNVMember" xlink:label="MPCEnergySolutionsNVMember" xlink:title="MPCEnergySolutionsNVMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="MPCCSIGmbHMember" xlink:to="MPCEnergySolutionsNVMember" xlink:title="definition: MPCCSIGmbHMember to MPCEnergySolutionsNVMember" order="3.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityMethodInvestmentsSummarizedConsolidatedFinancialInformationDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:label="ScheduleOfEquityMethodInvestmentsLineItems" xlink:title="ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAbstract" xlink:label="EquityMethodInvestmentSummarizedFinancialInformationAbstract" xlink:title="EquityMethodInvestmentSummarizedFinancialInformationAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentSummarizedFinancialInformationAbstract" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentSummarizedFinancialInformationAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent" xlink:label="AssetsCurrent" xlink:title="AssetsCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="AssetsCurrent" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to AssetsCurrent" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsNoncurrent" xlink:label="AssetsNoncurrent" xlink:title="AssetsNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="AssetsNoncurrent" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to AssetsNoncurrent" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent" xlink:label="LiabilitiesCurrent" xlink:title="LiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="LiabilitiesCurrent" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to LiabilitiesCurrent" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesNoncurrent" xlink:label="LiabilitiesNoncurrent" xlink:title="LiabilitiesNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="LiabilitiesNoncurrent" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to LiabilitiesNoncurrent" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentQuotedMarketValue" xlink:label="EquityMethodInvestmentQuotedMarketValue" xlink:title="EquityMethodInvestmentQuotedMarketValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentQuotedMarketValue" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentQuotedMarketValue" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Revenues" xlink:label="Revenues" xlink:title="Revenues" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="Revenues" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to Revenues" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="ProfitLoss" xlink:title="ProfitLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="ProfitLoss" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to ProfitLoss" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:label="ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:title="ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:label="ScheduleOfEquityMethodInvestmentsTable" xlink:title="ScheduleOfEquityMethodInvestmentsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="ScheduleOfEquityMethodInvestmentsTable" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to ScheduleOfEquityMethodInvestmentsTable" order="10.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis" xlink:label="OwnershipAxis" xlink:title="OwnershipAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfEquityMethodInvestmentsTable" xlink:to="OwnershipAxis" xlink:title="definition: ScheduleOfEquityMethodInvestmentsTable to OwnershipAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="OwnershipDomain" xlink:title="OwnershipDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="OwnershipAxis" xlink:to="OwnershipDomain" xlink:title="definition: OwnershipAxis to OwnershipDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="OwnershipDomain_2" xlink:title="OwnershipDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="OwnershipAxis" xlink:to="OwnershipDomain_2" xlink:title="definition: OwnershipAxis to OwnershipDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCContainerShipsASAMember" xlink:label="MPCContainerShipsASAMember" xlink:title="MPCContainerShipsASAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OwnershipDomain" xlink:to="MPCContainerShipsASAMember" xlink:title="definition: OwnershipDomain to MPCContainerShipsASAMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityMethodInvestmentsFairValuesOfAssetsAcquiredAndLiabilitiesAssumedDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:label="ScheduleOfEquityMethodInvestmentsLineItems" xlink:title="ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAbstract" xlink:label="EquityMethodInvestmentSummarizedFinancialInformationAbstract" xlink:title="EquityMethodInvestmentSummarizedFinancialInformationAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentSummarizedFinancialInformationAbstract" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentSummarizedFinancialInformationAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentsFairValueDisclosure" xlink:label="EquityMethodInvestmentsFairValueDisclosure" xlink:title="EquityMethodInvestmentsFairValueDisclosure" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentsFairValueDisclosure" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentsFairValueDisclosure" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets" xlink:label="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets" xlink:title="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" xlink:label="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" xlink:title="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:label="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:title="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:label="ScheduleOfEquityMethodInvestmentsTable" xlink:title="ScheduleOfEquityMethodInvestmentsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="ScheduleOfEquityMethodInvestmentsTable" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to ScheduleOfEquityMethodInvestmentsTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfEquityMethodInvestmentsTable" xlink:to="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="definition: ScheduleOfEquityMethodInvestmentsTable to ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain_2" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain_2" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCCSIGmbHMember" xlink:label="MPCCSIGmbHMember" xlink:title="MPCCSIGmbHMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCCSIGmbHMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCCSIGmbHMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LongtermDebtLongtermDebtDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="DebtInstrumentLineItems" xlink:title="DebtInstrumentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentsAbstract" xlink:label="DebtInstrumentsAbstract" xlink:title="DebtInstrumentsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentsAbstract" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentsAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtByCurrentAndNoncurrentAbstract" xlink:label="LongTermDebtByCurrentAndNoncurrentAbstract" xlink:title="LongTermDebtByCurrentAndNoncurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtByCurrentAndNoncurrentAbstract" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtByCurrentAndNoncurrentAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_LongTermDebtGrossCurrent" xlink:label="LongTermDebtGrossCurrent" xlink:title="LongTermDebtGrossCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtGrossCurrent" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtGrossCurrent" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsCurrentNet" xlink:label="DeferredFinanceCostsCurrentNet" xlink:title="DeferredFinanceCostsCurrentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DeferredFinanceCostsCurrentNet" xlink:title="definition: DebtInstrumentLineItems to DeferredFinanceCostsCurrentNet" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtCurrent" xlink:label="SecuredDebtCurrent" xlink:title="SecuredDebtCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="SecuredDebtCurrent" xlink:title="definition: DebtInstrumentLineItems to SecuredDebtCurrent" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_LongTermDebtGrossNoncurrent" xlink:label="LongTermDebtGrossNoncurrent" xlink:title="LongTermDebtGrossNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtGrossNoncurrent" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtGrossNoncurrent" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNoncurrentNet" xlink:label="DeferredFinanceCostsNoncurrentNet" xlink:title="DeferredFinanceCostsNoncurrentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DeferredFinanceCostsNoncurrentNet" xlink:title="definition: DebtInstrumentLineItems to DeferredFinanceCostsNoncurrentNet" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredLongTermDebt" xlink:label="SecuredLongTermDebt" xlink:title="SecuredLongTermDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="SecuredLongTermDebt" xlink:title="definition: DebtInstrumentLineItems to SecuredLongTermDebt" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="DebtInstrumentFaceAmount" xlink:title="DebtInstrumentFaceAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentFaceAmount" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentFaceAmount" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount" xlink:label="DebtInstrumentCarryingAmount" xlink:title="DebtInstrumentCarryingAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentCarryingAmount" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentCarryingAmount" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet" xlink:label="DeferredFinanceCostsNet" xlink:title="DeferredFinanceCostsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DeferredFinanceCostsNet" xlink:title="definition: DebtInstrumentLineItems to DeferredFinanceCostsNet" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_LongTermDebtIncludingAccruedInterest" xlink:label="LongTermDebtIncludingAccruedInterest" xlink:title="LongTermDebtIncludingAccruedInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtIncludingAccruedInterest" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtIncludingAccruedInterest" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="DebtInstrumentTable" xlink:title="DebtInstrumentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentTable" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentTable" order="13.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="DebtInstrumentAxis" xlink:title="DebtInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DebtInstrumentTable" xlink:to="DebtInstrumentAxis" xlink:title="definition: DebtInstrumentTable to DebtInstrumentAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain_2" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain_2" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_SeniorTermLoanFacilityAlphaBankSAMember" xlink:label="SeniorTermLoanFacilityAlphaBankSAMember" xlink:title="SeniorTermLoanFacilityAlphaBankSAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentNameDomain" xlink:to="SeniorTermLoanFacilityAlphaBankSAMember" xlink:title="definition: DebtInstrumentNameDomain to SeniorTermLoanFacilityAlphaBankSAMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TermLoanFacilityOstfriesischeVolksbankeGMember" xlink:label="TermLoanFacilityOstfriesischeVolksbankeGMember" xlink:title="TermLoanFacilityOstfriesischeVolksbankeGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentNameDomain" xlink:to="TermLoanFacilityOstfriesischeVolksbankeGMember" xlink:title="definition: DebtInstrumentNameDomain to TermLoanFacilityOstfriesischeVolksbankeGMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_CreditFacilityCommerzbankAktiengesellschaftMember" xlink:label="CreditFacilityCommerzbankAktiengesellschaftMember" xlink:title="CreditFacilityCommerzbankAktiengesellschaftMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentNameDomain" xlink:to="CreditFacilityCommerzbankAktiengesellschaftMember" xlink:title="definition: DebtInstrumentNameDomain to CreditFacilityCommerzbankAktiengesellschaftMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_SecondCreditFacilityCommerzbankAktiengesellschaftMember" xlink:label="SecondCreditFacilityCommerzbankAktiengesellschaftMember" xlink:title="SecondCreditFacilityCommerzbankAktiengesellschaftMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentNameDomain" xlink:to="SecondCreditFacilityCommerzbankAktiengesellschaftMember" xlink:title="definition: DebtInstrumentNameDomain to SecondCreditFacilityCommerzbankAktiengesellschaftMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DebtInstrumentTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: DebtInstrumentTable to RelatedPartyTransactionsByRelatedPartyAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="NonrelatedPartyMember" xlink:title="NonrelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="NonrelatedPartyMember" xlink:title="definition: RelatedPartyDomain to NonrelatedPartyMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LongtermDebt500MillionTermLoanFacilityDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="DebtInstrumentLineItems" xlink:title="DebtInstrumentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentsAbstract" xlink:label="DebtInstrumentsAbstract" xlink:title="DebtInstrumentsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentsAbstract" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentsAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfSecuredDebt" xlink:label="RepaymentsOfSecuredDebt" xlink:title="RepaymentsOfSecuredDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="RepaymentsOfSecuredDebt" xlink:title="definition: DebtInstrumentLineItems to RepaymentsOfSecuredDebt" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="DebtInstrumentFaceAmount" xlink:title="DebtInstrumentFaceAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentFaceAmount" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentFaceAmount" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfSubsidiariesProvidingSecurityForTermLoanFacility" xlink:label="NumberOfSubsidiariesProvidingSecurityForTermLoanFacility" xlink:title="NumberOfSubsidiariesProvidingSecurityForTermLoanFacility" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="NumberOfSubsidiariesProvidingSecurityForTermLoanFacility" xlink:title="definition: DebtInstrumentLineItems to NumberOfSubsidiariesProvidingSecurityForTermLoanFacility" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount" xlink:label="DebtInstrumentCarryingAmount" xlink:title="DebtInstrumentCarryingAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentCarryingAmount" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentCarryingAmount" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="DebtInstrumentTable" xlink:title="DebtInstrumentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentTable" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="DebtInstrumentAxis" xlink:title="DebtInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DebtInstrumentTable" xlink:to="DebtInstrumentAxis" xlink:title="definition: DebtInstrumentTable to DebtInstrumentAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain_2" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain_2" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_SeniorTermLoanFacilityAlphaBankSAMember" xlink:label="SeniorTermLoanFacilityAlphaBankSAMember" xlink:title="SeniorTermLoanFacilityAlphaBankSAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentNameDomain" xlink:to="SeniorTermLoanFacilityAlphaBankSAMember" xlink:title="definition: DebtInstrumentNameDomain to SeniorTermLoanFacilityAlphaBankSAMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LongtermDebtAnnualPrincipalPaymentsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="DebtInstrumentLineItems" xlink:title="DebtInstrumentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentsAbstract" xlink:label="DebtInstrumentsAbstract" xlink:title="DebtInstrumentsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentsAbstract" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentsAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtByMaturityAbstract" xlink:label="LongTermDebtByMaturityAbstract" xlink:title="LongTermDebtByMaturityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtByMaturityAbstract" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtByMaturityAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtWeightedAverageInterestRate" xlink:label="DebtWeightedAverageInterestRate" xlink:title="DebtWeightedAverageInterestRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtWeightedAverageInterestRate" xlink:title="definition: DebtInstrumentLineItems to DebtWeightedAverageInterestRate" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseLongTermDebt" xlink:label="InterestExpenseLongTermDebt" xlink:title="InterestExpenseLongTermDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="InterestExpenseLongTermDebt" xlink:title="definition: DebtInstrumentLineItems to InterestExpenseLongTermDebt" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextRollingTwelveMonths" xlink:label="LongTermDebtMaturitiesRepaymentsOfPrincipalInNextRollingTwelveMonths" xlink:title="LongTermDebtMaturitiesRepaymentsOfPrincipalInNextRollingTwelveMonths" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtMaturitiesRepaymentsOfPrincipalInNextRollingTwelveMonths" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtMaturitiesRepaymentsOfPrincipalInNextRollingTwelveMonths" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearTwo" xlink:label="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearTwo" xlink:title="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearTwo" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearTwo" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearTwo" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearThree" xlink:label="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearThree" xlink:title="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearThree" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearThree" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearThree" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFour" xlink:label="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFour" xlink:title="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFour" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFour" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFour" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFiveAndThereafter" xlink:label="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFiveAndThereafter" xlink:title="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFiveAndThereafter" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFiveAndThereafter" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtMaturitiesRepaymentsOfPrincipalInRollingYearFiveAndThereafter" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt" xlink:label="LongTermDebt" xlink:title="LongTermDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebt" xlink:title="definition: DebtInstrumentLineItems to LongTermDebt" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscount" xlink:label="DebtInstrumentUnamortizedDiscount" xlink:title="DebtInstrumentUnamortizedDiscount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentUnamortizedDiscount" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentUnamortizedDiscount" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_LongTermDebtGrossCurrent" xlink:label="LongTermDebtGrossCurrent" xlink:title="LongTermDebtGrossCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtGrossCurrent" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtGrossCurrent" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_LongTermDebtGrossNoncurrent" xlink:label="LongTermDebtGrossNoncurrent" xlink:title="LongTermDebtGrossNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="LongTermDebtGrossNoncurrent" xlink:title="definition: DebtInstrumentLineItems to LongTermDebtGrossNoncurrent" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="DebtInstrumentTable" xlink:title="DebtInstrumentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentTable" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentTable" order="14.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DebtInstrumentTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: DebtInstrumentTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="NonrelatedPartyMember" xlink:title="NonrelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="NonrelatedPartyMember" xlink:title="definition: RelatedPartyDomain to NonrelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="DebtInstrumentAxis" xlink:title="DebtInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DebtInstrumentTable" xlink:to="DebtInstrumentAxis" xlink:title="definition: DebtInstrumentTable to DebtInstrumentAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain_2" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain_2" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ThirdPartyDebtMember" xlink:label="ThirdPartyDebtMember" xlink:title="ThirdPartyDebtMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentNameDomain" xlink:to="ThirdPartyDebtMember" xlink:title="definition: DebtInstrumentNameDomain to ThirdPartyDebtMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LongtermDebtAndFinancialLiabilitiesFinancialLiabilitiesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="DebtInstrumentLineItems" xlink:title="DebtInstrumentLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FinancialLiabilitiesAbstract" xlink:label="FinancialLiabilitiesAbstract" xlink:title="FinancialLiabilitiesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="FinancialLiabilitiesAbstract" xlink:title="definition: DebtInstrumentLineItems to FinancialLiabilitiesAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FinancialLiabilitiesByCurrentAndNoncurrentAbstract" xlink:label="FinancialLiabilitiesByCurrentAndNoncurrentAbstract" xlink:title="FinancialLiabilitiesByCurrentAndNoncurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="FinancialLiabilitiesByCurrentAndNoncurrentAbstract" xlink:title="definition: DebtInstrumentLineItems to FinancialLiabilitiesByCurrentAndNoncurrentAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FinancialLiabilitiesGrossCurrent" xlink:label="FinancialLiabilitiesGrossCurrent" xlink:title="FinancialLiabilitiesGrossCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="FinancialLiabilitiesGrossCurrent" xlink:title="definition: DebtInstrumentLineItems to FinancialLiabilitiesGrossCurrent" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DeferredFinanceCostsCurrentNetFinancialLiabilities" xlink:label="DeferredFinanceCostsCurrentNetFinancialLiabilities" xlink:title="DeferredFinanceCostsCurrentNetFinancialLiabilities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DeferredFinanceCostsCurrentNetFinancialLiabilities" xlink:title="definition: DebtInstrumentLineItems to DeferredFinanceCostsCurrentNetFinancialLiabilities" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLongTermDebtCurrent" xlink:label="OtherLongTermDebtCurrent" xlink:title="OtherLongTermDebtCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="OtherLongTermDebtCurrent" xlink:title="definition: DebtInstrumentLineItems to OtherLongTermDebtCurrent" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FinancialLiabilitiesGrossNoncurrent" xlink:label="FinancialLiabilitiesGrossNoncurrent" xlink:title="FinancialLiabilitiesGrossNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="FinancialLiabilitiesGrossNoncurrent" xlink:title="definition: DebtInstrumentLineItems to FinancialLiabilitiesGrossNoncurrent" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DeferredFinanceCostsNoncurrentNetFinancialLiabilities" xlink:label="DeferredFinanceCostsNoncurrentNetFinancialLiabilities" xlink:title="DeferredFinanceCostsNoncurrentNetFinancialLiabilities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DeferredFinanceCostsNoncurrentNetFinancialLiabilities" xlink:title="definition: DebtInstrumentLineItems to DeferredFinanceCostsNoncurrentNetFinancialLiabilities" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLongTermDebtNoncurrent" xlink:label="OtherLongTermDebtNoncurrent" xlink:title="OtherLongTermDebtNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="OtherLongTermDebtNoncurrent" xlink:title="definition: DebtInstrumentLineItems to OtherLongTermDebtNoncurrent" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FinancialLiabilitiesCarryingAmount" xlink:label="FinancialLiabilitiesCarryingAmount" xlink:title="FinancialLiabilitiesCarryingAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="FinancialLiabilitiesCarryingAmount" xlink:title="definition: DebtInstrumentLineItems to FinancialLiabilitiesCarryingAmount" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DeferredFinancingLiabilitiesCostsNet" xlink:label="DeferredFinancingLiabilitiesCostsNet" xlink:title="DeferredFinancingLiabilitiesCostsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DeferredFinancingLiabilitiesCostsNet" xlink:title="definition: DebtInstrumentLineItems to DeferredFinancingLiabilitiesCostsNet" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FinancialLiabilities" xlink:label="FinancialLiabilities" xlink:title="FinancialLiabilities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="FinancialLiabilities" xlink:title="definition: DebtInstrumentLineItems to FinancialLiabilities" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="DebtInstrumentTable" xlink:title="DebtInstrumentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentTable" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentTable" order="12.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DebtInstrumentTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: DebtInstrumentTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVMagicThunderMember" xlink:label="MVMagicThunderMember" xlink:title="MVMagicThunderMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="MVMagicThunderMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to MVMagicThunderMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVMagicPerseusMember" xlink:label="MVMagicPerseusMember" xlink:title="MVMagicPerseusMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="MVMagicPerseusMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to MVMagicPerseusMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LongtermDebtSaleAndLeasebackAgreementsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="DebtInstrumentLineItems" xlink:title="DebtInstrumentLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_SaleAndLeasebackArrangementsAbstract" xlink:label="SaleAndLeasebackArrangementsAbstract" xlink:title="SaleAndLeasebackArrangementsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="SaleAndLeasebackArrangementsAbstract" xlink:title="definition: DebtInstrumentLineItems to SaleAndLeasebackArrangementsAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NumberOfSaleAndLeasebackArrangements" xlink:label="NumberOfSaleAndLeasebackArrangements" xlink:title="NumberOfSaleAndLeasebackArrangements" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="NumberOfSaleAndLeasebackArrangements" xlink:title="definition: DebtInstrumentLineItems to NumberOfSaleAndLeasebackArrangements" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1" xlink:label="FinanceLeaseWeightedAverageRemainingLeaseTerm1" xlink:title="FinanceLeaseWeightedAverageRemainingLeaseTerm1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="FinanceLeaseWeightedAverageRemainingLeaseTerm1" xlink:title="definition: DebtInstrumentLineItems to FinanceLeaseWeightedAverageRemainingLeaseTerm1" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtWeightedAverageInterestRate" xlink:label="DebtWeightedAverageInterestRate" xlink:title="DebtWeightedAverageInterestRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="DebtWeightedAverageInterestRate" xlink:title="definition: DebtInstrumentLineItems to DebtWeightedAverageInterestRate" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="InterestExpenseNonoperating" xlink:title="InterestExpenseNonoperating" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentLineItems" xlink:to="InterestExpenseNonoperating" xlink:title="definition: DebtInstrumentLineItems to InterestExpenseNonoperating" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="DebtInstrumentTable" xlink:title="DebtInstrumentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="DebtInstrumentLineItems" xlink:to="DebtInstrumentTable" xlink:title="definition: DebtInstrumentLineItems to DebtInstrumentTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="PropertyPlantAndEquipmentByTypeAxis" xlink:title="PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DebtInstrumentTable" xlink:to="PropertyPlantAndEquipmentByTypeAxis" xlink:title="definition: DebtInstrumentTable to PropertyPlantAndEquipmentByTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="PropertyPlantAndEquipmentByTypeAxis" xlink:to="PropertyPlantAndEquipmentTypeDomain_2" xlink:title="definition: PropertyPlantAndEquipmentByTypeAxis to PropertyPlantAndEquipmentTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVMagicThunderAndMVMagicPerseusMember" xlink:label="MVMagicThunderAndMVMagicPerseusMember" xlink:title="MVMagicThunderAndMVMagicPerseusMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="MVMagicThunderAndMVMagicPerseusMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to MVMagicThunderAndMVMagicPerseusMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVMagicThunderMember" xlink:label="MVMagicThunderMember" xlink:title="MVMagicThunderMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="MVMagicThunderAndMVMagicPerseusMember" xlink:to="MVMagicThunderMember" xlink:title="definition: MVMagicThunderAndMVMagicPerseusMember to MVMagicThunderMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MVMagicPerseusMember" xlink:label="MVMagicPerseusMember" xlink:title="MVMagicPerseusMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="MVMagicThunderAndMVMagicPerseusMember" xlink:to="MVMagicPerseusMember" xlink:title="definition: MVMagicThunderAndMVMagicPerseusMember to MVMagicPerseusMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LongtermDebtAndFinancialLiabilitiesAnnualFinancialLiabilitiesDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InvestmentInEquitySecuritiesEquityInvestmentsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:label="ScheduleOfEquityMethodInvestmentsLineItems" xlink:title="ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiAndWithoutReadilyDeterminableFairValueAbstract" xlink:label="EquitySecuritiesFvNiAndWithoutReadilyDeterminableFairValueAbstract" xlink:title="EquitySecuritiesFvNiAndWithoutReadilyDeterminableFairValueAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesFvNiAndWithoutReadilyDeterminableFairValueAbstract" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesFvNiAndWithoutReadilyDeterminableFairValueAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValueAmount" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquitySecuritiesWithoutReadilyDeterminableFairValueTransfers" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValueTransfers" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValueTransfers" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValueTransfers" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValueTransfers" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquitySecuritiesWithoutReadilyDeterminableFairValuePurchased" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValuePurchased" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValuePurchased" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValuePurchased" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValuePurchased" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquitySecuritiesWithoutReadilyDeterminableFairValueProceedsFromSaleOfEquitySecurities" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValueProceedsFromSaleOfEquitySecurities" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValueProceedsFromSaleOfEquitySecurities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValueProceedsFromSaleOfEquitySecurities" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValueProceedsFromSaleOfEquitySecurities" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquitySecuritiesWithoutReadilyDeterminableFairValueDistributions" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValueDistributions" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValueDistributions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValueDistributions" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValueDistributions" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLoss" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLoss" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLoss" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLoss" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquitySecuritiesWithoutReadilyDeterminableFairValueUnrealizedForeignExchangeGainLoss" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValueUnrealizedForeignExchangeGainLoss" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValueUnrealizedForeignExchangeGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValueUnrealizedForeignExchangeGainLoss" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValueUnrealizedForeignExchangeGainLoss" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsTransfersOther" xlink:label="EquityMethodInvestmentsTransfersOther" xlink:title="EquityMethodInvestmentsTransfersOther" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentsTransfersOther" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentsTransfersOther" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentPercentageOfIncreaseInEquityInvestment" xlink:label="EquityMethodInvestmentPercentageOfIncreaseInEquityInvestment" xlink:title="EquityMethodInvestmentPercentageOfIncreaseInEquityInvestment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentPercentageOfIncreaseInEquityInvestment" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentPercentageOfIncreaseInEquityInvestment" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentDistributionReceived" xlink:label="EquityMethodInvestmentDistributionReceived" xlink:title="EquityMethodInvestmentDistributionReceived" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquityMethodInvestmentDistributionReceived" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquityMethodInvestmentDistributionReceived" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquitySecuritiesWithoutReadilyDeterminableFairValueDividendIncome" xlink:label="EquitySecuritiesWithoutReadilyDeterminableFairValueDividendIncome" xlink:title="EquitySecuritiesWithoutReadilyDeterminableFairValueDividendIncome" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="EquitySecuritiesWithoutReadilyDeterminableFairValueDividendIncome" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to EquitySecuritiesWithoutReadilyDeterminableFairValueDividendIncome" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:label="ScheduleOfEquityMethodInvestmentsTable" xlink:title="ScheduleOfEquityMethodInvestmentsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="ScheduleOfEquityMethodInvestmentsTable" xlink:title="definition: ScheduleOfEquityMethodInvestmentsLineItems to ScheduleOfEquityMethodInvestmentsTable" order="13.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfEquityMethodInvestmentsTable" xlink:to="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="definition: ScheduleOfEquityMethodInvestmentsTable to ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain_2" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain_2" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCMnchmeyerPetersenITServicesGmbHMember" xlink:label="MPCMnchmeyerPetersenITServicesGmbHMember" xlink:title="MPCMnchmeyerPetersenITServicesGmbHMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCMnchmeyerPetersenITServicesGmbHMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCMnchmeyerPetersenITServicesGmbHMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCEuropeanCleanEnergySAMember" xlink:label="MPCEuropeanCleanEnergySAMember" xlink:title="MPCEuropeanCleanEnergySAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCEuropeanCleanEnergySAMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCEuropeanCleanEnergySAMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCOSEOffshoreKSMember" xlink:label="MPCOSEOffshoreKSMember" xlink:title="MPCOSEOffshoreKSMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCOSEOffshoreKSMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCOSEOffshoreKSMember" order="3.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InvestmentInDebtSecuritiesHeldToMaturityInvestmentInDebtSecuritiesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:label="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:title="ScheduleOfHeldToMaturitySecuritiesLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesAbstract" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesAbstract" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityFairValueCurrent" xlink:label="DebtSecuritiesHeldToMaturityFairValueCurrent" xlink:title="DebtSecuritiesHeldToMaturityFairValueCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="DebtSecuritiesHeldToMaturityFairValueCurrent" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to DebtSecuritiesHeldToMaturityFairValueCurrent" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossUnrealized" xlink:label="ForeignCurrencyTransactionGainLossUnrealized" xlink:title="ForeignCurrencyTransactionGainLossUnrealized" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="ForeignCurrencyTransactionGainLossUnrealized" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to ForeignCurrencyTransactionGainLossUnrealized" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss" xlink:label="DebtSecuritiesHeldToMaturityAllowanceForCreditLoss" xlink:title="DebtSecuritiesHeldToMaturityAllowanceForCreditLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="DebtSecuritiesHeldToMaturityAllowanceForCreditLoss" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to DebtSecuritiesHeldToMaturityAllowanceForCreditLoss" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DebtSecuritiesHeldToMaturityUnrealizedGainsAbstract" xlink:label="DebtSecuritiesHeldToMaturityUnrealizedGainsAbstract" xlink:title="DebtSecuritiesHeldToMaturityUnrealizedGainsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="DebtSecuritiesHeldToMaturityUnrealizedGainsAbstract" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to DebtSecuritiesHeldToMaturityUnrealizedGainsAbstract" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateYearOne" xlink:label="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateYearOne" xlink:title="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateYearOne" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateYearOne" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateYearOne" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive" xlink:label="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive" xlink:title="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYear5Through10" xlink:label="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYear5Through10" xlink:title="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYear5Through10" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYear5Through10" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to DebtSecuritiesHeldToMaturityUnrealizedGainsMaturityAllocatedAndSingleMaturityDateAfterYear5Through10" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain" xlink:label="HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain" xlink:title="HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue" xlink:label="HeldToMaturitySecuritiesFairValue" xlink:title="HeldToMaturitySecuritiesFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesFairValue" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesFairValue" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount" order="16.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount" order="17.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount" xlink:label="HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount" xlink:title="HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount" order="18.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss" xlink:label="DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss" xlink:title="DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss" order="19.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:label="ScheduleOfHeldToMaturitySecuritiesTable" xlink:title="ScheduleOfHeldToMaturitySecuritiesTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ScheduleOfHeldToMaturitySecuritiesLineItems" xlink:to="ScheduleOfHeldToMaturitySecuritiesTable" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesLineItems to ScheduleOfHeldToMaturitySecuritiesTable" order="20.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="FairValueByAssetClassAxis" xlink:title="FairValueByAssetClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfHeldToMaturitySecuritiesTable" xlink:to="FairValueByAssetClassAxis" xlink:title="definition: ScheduleOfHeldToMaturitySecuritiesTable to FairValueByAssetClassAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:title="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByAssetClassAxis" xlink:to="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:title="definition: FairValueByAssetClassAxis to FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" xlink:title="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByAssetClassAxis" xlink:to="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" xlink:title="definition: FairValueByAssetClassAxis to FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesMember" xlink:label="DebtSecuritiesMember" xlink:title="DebtSecuritiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:to="DebtSecuritiesMember" xlink:title="definition: FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain to DebtSecuritiesMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InvestmentInDebtSecuritiesCarryingAmountAndUnrealizedGainsOfTradingDebtSecuritiesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems" xlink:label="DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems" xlink:title="DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DebtSecuritiesCarryingAmountAndUnrealizedGainsAbstract" xlink:label="DebtSecuritiesCarryingAmountAndUnrealizedGainsAbstract" xlink:title="DebtSecuritiesCarryingAmountAndUnrealizedGainsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems" xlink:to="DebtSecuritiesCarryingAmountAndUnrealizedGainsAbstract" xlink:title="definition: DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems to DebtSecuritiesCarryingAmountAndUnrealizedGainsAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradingSecuritiesDebt" xlink:label="TradingSecuritiesDebt" xlink:title="TradingSecuritiesDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems" xlink:to="TradingSecuritiesDebt" xlink:title="definition: DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems to TradingSecuritiesDebt" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesTradingGainLoss" xlink:label="DebtSecuritiesTradingGainLoss" xlink:title="DebtSecuritiesTradingGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems" xlink:to="DebtSecuritiesTradingGainLoss" xlink:title="definition: DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems to DebtSecuritiesTradingGainLoss" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLossTable" xlink:label="DebtSecuritiesHeldToMaturityAllowanceForCreditLossTable" xlink:title="DebtSecuritiesHeldToMaturityAllowanceForCreditLossTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems" xlink:to="DebtSecuritiesHeldToMaturityAllowanceForCreditLossTable" xlink:title="definition: DebtSecuritiesHeldtomaturityAllowanceForCreditLossLineItems to DebtSecuritiesHeldToMaturityAllowanceForCreditLossTable" order="4.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="FinancialInstrumentAxis" xlink:title="FinancialInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DebtSecuritiesHeldToMaturityAllowanceForCreditLossTable" xlink:to="FinancialInstrumentAxis" xlink:title="definition: DebtSecuritiesHeldToMaturityAllowanceForCreditLossTable to FinancialInstrumentAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:title="TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FinancialInstrumentAxis" xlink:to="TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:title="definition: FinancialInstrumentAxis to TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_2" xlink:title="TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FinancialInstrumentAxis" xlink:to="TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_2" xlink:title="definition: FinancialInstrumentAxis to TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="CorporateDebtSecuritiesMember" xlink:title="CorporateDebtSecuritiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="CorporateDebtSecuritiesMember" xlink:title="definition: TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain to CorporateDebtSecuritiesMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityCapitalStructureCapitalStockDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EquityCapitalStructureSeriesDCumulativePerpetualConvertiblePreferredSharesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems" xlink:label="ClassOfStockLineItems" xlink:title="ClassOfStockLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityAbstract" xlink:label="TemporaryEquityAbstract" xlink:title="TemporaryEquityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="TemporaryEquityAbstract" xlink:title="definition: ClassOfStockLineItems to TemporaryEquityAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncontrollingInterestAbstract" xlink:label="NoncontrollingInterestAbstract" xlink:title="NoncontrollingInterestAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="NoncontrollingInterestAbstract" xlink:title="definition: ClassOfStockLineItems to NoncontrollingInterestAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:label="MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:title="MinorityInterestOwnershipPercentageByNoncontrollingOwners" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:title="definition: ClassOfStockLineItems to MinorityInterestOwnershipPercentageByNoncontrollingOwners" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent" xlink:label="MinorityInterestOwnershipPercentageByParent" xlink:title="MinorityInterestOwnershipPercentageByParent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="MinorityInterestOwnershipPercentageByParent" xlink:title="definition: ClassOfStockLineItems to MinorityInterestOwnershipPercentageByParent" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_NoncontrollingInterestFromFirstTimeConsolidationEffects" xlink:label="NoncontrollingInterestFromFirstTimeConsolidationEffects" xlink:title="NoncontrollingInterestFromFirstTimeConsolidationEffects" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="NoncontrollingInterestFromFirstTimeConsolidationEffects" xlink:title="definition: ClassOfStockLineItems to NoncontrollingInterestFromFirstTimeConsolidationEffects" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" xlink:label="IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" xlink:title="IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" xlink:title="definition: ClassOfStockLineItems to IncreaseDecreaseInEquityFromChangesInNoncontrollingInterest" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToMinorityShareholders" xlink:label="PaymentsToMinorityShareholders" xlink:title="PaymentsToMinorityShareholders" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="PaymentsToMinorityShareholders" xlink:title="definition: ClassOfStockLineItems to PaymentsToMinorityShareholders" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromMinorityShareholders" xlink:label="ProceedsFromMinorityShareholders" xlink:title="ProceedsFromMinorityShareholders" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="ProceedsFromMinorityShareholders" xlink:title="definition: ClassOfStockLineItems to ProceedsFromMinorityShareholders" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="PreferredStockDividendRatePercentage" xlink:title="PreferredStockDividendRatePercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="PreferredStockDividendRatePercentage" xlink:title="definition: ClassOfStockLineItems to PreferredStockDividendRatePercentage" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesIssued" xlink:label="TemporaryEquitySharesIssued" xlink:title="TemporaryEquitySharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="TemporaryEquitySharesIssued" xlink:title="definition: ClassOfStockLineItems to TemporaryEquitySharesIssued" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PreferredStockStatedValuePerShare" xlink:label="PreferredStockStatedValuePerShare" xlink:title="PreferredStockStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="PreferredStockStatedValuePerShare" xlink:title="definition: ClassOfStockLineItems to PreferredStockStatedValuePerShare" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityParOrStatedValuePerShare" xlink:label="TemporaryEquityParOrStatedValuePerShare" xlink:title="TemporaryEquityParOrStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="TemporaryEquityParOrStatedValuePerShare" xlink:title="definition: ClassOfStockLineItems to TemporaryEquityParOrStatedValuePerShare" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:label="ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:title="ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:title="definition: ClassOfStockLineItems to ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PreferredStockEffectiveInterestRate" xlink:label="PreferredStockEffectiveInterestRate" xlink:title="PreferredStockEffectiveInterestRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="PreferredStockEffectiveInterestRate" xlink:title="definition: ClassOfStockLineItems to PreferredStockEffectiveInterestRate" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PreferredStockExpectedLife" xlink:label="PreferredStockExpectedLife" xlink:title="PreferredStockExpectedLife" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="PreferredStockExpectedLife" xlink:title="definition: ClassOfStockLineItems to PreferredStockExpectedLife" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:label="PreferredStockDividendsIncomeStatementImpact" xlink:title="PreferredStockDividendsIncomeStatementImpact" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="PreferredStockDividendsIncomeStatementImpact" xlink:title="definition: ClassOfStockLineItems to PreferredStockDividendsIncomeStatementImpact" order="16.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent" xlink:label="TemporaryEquityCarryingAmountAttributableToParent" xlink:title="TemporaryEquityCarryingAmountAttributableToParent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="TemporaryEquityCarryingAmountAttributableToParent" xlink:title="definition: ClassOfStockLineItems to TemporaryEquityCarryingAmountAttributableToParent" order="17.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsPayableCurrent" xlink:label="DividendsPayableCurrent" xlink:title="DividendsPayableCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockLineItems" xlink:to="DividendsPayableCurrent" xlink:title="definition: ClassOfStockLineItems to DividendsPayableCurrent" order="18.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="ScheduleOfStockByClassTable" xlink:title="ScheduleOfStockByClassTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ClassOfStockLineItems" xlink:to="ScheduleOfStockByClassTable" xlink:title="definition: ClassOfStockLineItems to ScheduleOfStockByClassTable" order="19.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="StatementClassOfStockAxis" xlink:title="StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfStockByClassTable" xlink:to="StatementClassOfStockAxis" xlink:title="definition: ScheduleOfStockByClassTable to StatementClassOfStockAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain_2" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain_2" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesDPreferredStockMember" xlink:label="SeriesDPreferredStockMember" xlink:title="SeriesDPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesDPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesDPreferredStockMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfStockByClassTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfStockByClassTable to RelatedPartyTransactionsByRelatedPartyAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="CounterpartyNameAxis" xlink:title="CounterpartyNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfStockByClassTable" xlink:to="CounterpartyNameAxis" xlink:title="definition: ScheduleOfStockByClassTable to CounterpartyNameAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="RepurchaseAgreementCounterpartyNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="CounterpartyNameAxis" xlink:to="RepurchaseAgreementCounterpartyNameDomain_2" xlink:title="definition: CounterpartyNameAxis to RepurchaseAgreementCounterpartyNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ToroCorpMember" xlink:label="ToroCorpMember" xlink:title="ToroCorpMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RepurchaseAgreementCounterpartyNameDomain" xlink:to="ToroCorpMember" xlink:title="definition: RepurchaseAgreementCounterpartyNameDomain to ToroCorpMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis" xlink:label="OwnershipAxis" xlink:title="OwnershipAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfStockByClassTable" xlink:to="OwnershipAxis" xlink:title="definition: ScheduleOfStockByClassTable to OwnershipAxis" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="OwnershipDomain" xlink:title="OwnershipDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="OwnershipAxis" xlink:to="OwnershipDomain" xlink:title="definition: OwnershipAxis to OwnershipDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="OwnershipDomain_2" xlink:title="OwnershipDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="OwnershipAxis" xlink:to="OwnershipDomain_2" xlink:title="definition: OwnershipAxis to OwnershipDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCCSIGmbHMember" xlink:label="MPCCSIGmbHMember" xlink:title="MPCCSIGmbHMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OwnershipDomain" xlink:to="MPCCSIGmbHMember" xlink:title="definition: OwnershipDomain to MPCCSIGmbHMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ParqueSolarLaPerlaSociedadAnonimaDeMember" xlink:label="ParqueSolarLaPerlaSociedadAnonimaDeMember" xlink:title="ParqueSolarLaPerlaSociedadAnonimaDeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="OwnershipDomain" xlink:to="ParqueSolarLaPerlaSociedadAnonimaDeMember" xlink:title="definition: OwnershipDomain to ParqueSolarLaPerlaSociedadAnonimaDeMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosuresInvestmentInDebtSecuritiesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:title="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsFairValueDisclosureAbstract" xlink:label="InvestmentsFairValueDisclosureAbstract" xlink:title="InvestmentsFairValueDisclosureAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="InvestmentsFairValueDisclosureAbstract" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to InvestmentsFairValueDisclosureAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue" xlink:label="HeldToMaturitySecuritiesFairValue" xlink:title="HeldToMaturitySecuritiesFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="HeldToMaturitySecuritiesFairValue" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to HeldToMaturitySecuritiesFairValue" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradingSecuritiesDebt" xlink:label="TradingSecuritiesDebt" xlink:title="TradingSecuritiesDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="TradingSecuritiesDebt" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to TradingSecuritiesDebt" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsFairValueDisclosure" xlink:label="InvestmentsFairValueDisclosure" xlink:title="InvestmentsFairValueDisclosure" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="InvestmentsFairValueDisclosure" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to InvestmentsFairValueDisclosure" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_FairValueImpairmentLoss" xlink:label="FairValueImpairmentLoss" xlink:title="FairValueImpairmentLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="FairValueImpairmentLoss" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to FairValueImpairmentLoss" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:title="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="FairValueByMeasurementBasisAxis" xlink:title="FairValueByMeasurementBasisAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="FairValueByMeasurementBasisAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to FairValueByMeasurementBasisAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosureItemAmountsDomain" xlink:label="FairValueDisclosureItemAmountsDomain" xlink:title="FairValueDisclosureItemAmountsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByMeasurementBasisAxis" xlink:to="FairValueDisclosureItemAmountsDomain" xlink:title="definition: FairValueByMeasurementBasisAxis to FairValueDisclosureItemAmountsDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="PortionAtFairValueFairValueDisclosureMember" xlink:title="PortionAtFairValueFairValueDisclosureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueDisclosureItemAmountsDomain" xlink:to="PortionAtFairValueFairValueDisclosureMember" xlink:title="definition: FairValueDisclosureItemAmountsDomain to PortionAtFairValueFairValueDisclosureMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="PortionAtFairValueFairValueDisclosureMember_2" xlink:title="PortionAtFairValueFairValueDisclosureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByMeasurementBasisAxis" xlink:to="PortionAtFairValueFairValueDisclosureMember_2" xlink:title="definition: FairValueByMeasurementBasisAxis to PortionAtFairValueFairValueDisclosureMember_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember" xlink:label="CarryingReportedAmountFairValueDisclosureMember" xlink:title="CarryingReportedAmountFairValueDisclosureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueDisclosureItemAmountsDomain" xlink:to="CarryingReportedAmountFairValueDisclosureMember" xlink:title="definition: FairValueDisclosureItemAmountsDomain to CarryingReportedAmountFairValueDisclosureMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:label="EstimateOfFairValueFairValueDisclosureMember" xlink:title="EstimateOfFairValueFairValueDisclosureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueDisclosureItemAmountsDomain" xlink:to="EstimateOfFairValueFairValueDisclosureMember" xlink:title="definition: FairValueDisclosureItemAmountsDomain to EstimateOfFairValueFairValueDisclosureMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="FairValueByFairValueHierarchyLevelAxis" xlink:title="FairValueByFairValueHierarchyLevelAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="FairValueByFairValueHierarchyLevelAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to FairValueByFairValueHierarchyLevelAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="FairValueMeasurementsFairValueHierarchyDomain" xlink:title="FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByFairValueHierarchyLevelAxis" xlink:to="FairValueMeasurementsFairValueHierarchyDomain" xlink:title="definition: FairValueByFairValueHierarchyLevelAxis to FairValueMeasurementsFairValueHierarchyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="FairValueMeasurementsFairValueHierarchyDomain_2" xlink:title="FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByFairValueHierarchyLevelAxis" xlink:to="FairValueMeasurementsFairValueHierarchyDomain_2" xlink:title="definition: FairValueByFairValueHierarchyLevelAxis to FairValueMeasurementsFairValueHierarchyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="FairValueInputsLevel1Member" xlink:title="FairValueInputsLevel1Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueMeasurementsFairValueHierarchyDomain" xlink:to="FairValueInputsLevel1Member" xlink:title="definition: FairValueMeasurementsFairValueHierarchyDomain to FairValueInputsLevel1Member" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosuresDerivativeContractsMeasuredOnRecurringBasisDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:title="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetsAbstract" xlink:label="DerivativeAssetsAbstract" xlink:title="DerivativeAssetsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="DerivativeAssetsAbstract" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to DerivativeAssetsAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitiesAbstract" xlink:label="DerivativeLiabilitiesAbstract" xlink:title="DerivativeLiabilitiesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="DerivativeLiabilitiesAbstract" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to DerivativeLiabilitiesAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure" xlink:label="ForeignCurrencyContractsLiabilityFairValueDisclosure" xlink:title="ForeignCurrencyContractsLiabilityFairValueDisclosure" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="ForeignCurrencyContractsLiabilityFairValueDisclosure" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to ForeignCurrencyContractsLiabilityFairValueDisclosure" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyContractAssetFairValueDisclosure" xlink:label="ForeignCurrencyContractAssetFairValueDisclosure" xlink:title="ForeignCurrencyContractAssetFairValueDisclosure" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="ForeignCurrencyContractAssetFairValueDisclosure" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to ForeignCurrencyContractAssetFairValueDisclosure" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:title="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" order="5.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="FairValueByMeasurementFrequencyAxis" xlink:title="FairValueByMeasurementFrequencyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="FairValueByMeasurementFrequencyAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to FairValueByMeasurementFrequencyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="FairValueMeasurementFrequencyDomain" xlink:title="FairValueMeasurementFrequencyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByMeasurementFrequencyAxis" xlink:to="FairValueMeasurementFrequencyDomain" xlink:title="definition: FairValueByMeasurementFrequencyAxis to FairValueMeasurementFrequencyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="FairValueMeasurementFrequencyDomain_2" xlink:title="FairValueMeasurementFrequencyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByMeasurementFrequencyAxis" xlink:to="FairValueMeasurementFrequencyDomain_2" xlink:title="definition: FairValueByMeasurementFrequencyAxis to FairValueMeasurementFrequencyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="FairValueMeasurementsRecurringMember" xlink:title="FairValueMeasurementsRecurringMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueMeasurementFrequencyDomain" xlink:to="FairValueMeasurementsRecurringMember" xlink:title="definition: FairValueMeasurementFrequencyDomain to FairValueMeasurementsRecurringMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="FairValueByFairValueHierarchyLevelAxis" xlink:title="FairValueByFairValueHierarchyLevelAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="FairValueByFairValueHierarchyLevelAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to FairValueByFairValueHierarchyLevelAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="FairValueMeasurementsFairValueHierarchyDomain" xlink:title="FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByFairValueHierarchyLevelAxis" xlink:to="FairValueMeasurementsFairValueHierarchyDomain" xlink:title="definition: FairValueByFairValueHierarchyLevelAxis to FairValueMeasurementsFairValueHierarchyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="FairValueMeasurementsFairValueHierarchyDomain_2" xlink:title="FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByFairValueHierarchyLevelAxis" xlink:to="FairValueMeasurementsFairValueHierarchyDomain_2" xlink:title="definition: FairValueByFairValueHierarchyLevelAxis to FairValueMeasurementsFairValueHierarchyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="FairValueInputsLevel2Member" xlink:title="FairValueInputsLevel2Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueMeasurementsFairValueHierarchyDomain" xlink:to="FairValueInputsLevel2Member" xlink:title="definition: FairValueMeasurementsFairValueHierarchyDomain to FairValueInputsLevel2Member" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="FairValueByMeasurementBasisAxis" xlink:title="FairValueByMeasurementBasisAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="FairValueByMeasurementBasisAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to FairValueByMeasurementBasisAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosureItemAmountsDomain" xlink:label="FairValueDisclosureItemAmountsDomain" xlink:title="FairValueDisclosureItemAmountsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByMeasurementBasisAxis" xlink:to="FairValueDisclosureItemAmountsDomain" xlink:title="definition: FairValueByMeasurementBasisAxis to FairValueDisclosureItemAmountsDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="PortionAtFairValueFairValueDisclosureMember" xlink:title="PortionAtFairValueFairValueDisclosureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueDisclosureItemAmountsDomain" xlink:to="PortionAtFairValueFairValueDisclosureMember" xlink:title="definition: FairValueDisclosureItemAmountsDomain to PortionAtFairValueFairValueDisclosureMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="PortionAtFairValueFairValueDisclosureMember_2" xlink:title="PortionAtFairValueFairValueDisclosureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByMeasurementBasisAxis" xlink:to="PortionAtFairValueFairValueDisclosureMember_2" xlink:title="definition: FairValueByMeasurementBasisAxis to PortionAtFairValueFairValueDisclosureMember_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:label="EstimateOfFairValueFairValueDisclosureMember" xlink:title="EstimateOfFairValueFairValueDisclosureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueDisclosureItemAmountsDomain" xlink:to="EstimateOfFairValueFairValueDisclosureMember" xlink:title="definition: FairValueDisclosureItemAmountsDomain to EstimateOfFairValueFairValueDisclosureMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember" xlink:label="CarryingReportedAmountFairValueDisclosureMember" xlink:title="CarryingReportedAmountFairValueDisclosureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueDisclosureItemAmountsDomain" xlink:to="CarryingReportedAmountFairValueDisclosureMember" xlink:title="definition: FairValueDisclosureItemAmountsDomain to CarryingReportedAmountFairValueDisclosureMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingDesignationAxis" xlink:label="HedgingDesignationAxis" xlink:title="HedgingDesignationAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="HedgingDesignationAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to HedgingDesignationAxis" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingDesignationDomain" xlink:label="HedgingDesignationDomain" xlink:title="HedgingDesignationDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="HedgingDesignationAxis" xlink:to="HedgingDesignationDomain" xlink:title="definition: HedgingDesignationAxis to HedgingDesignationDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingDesignationDomain" xlink:label="HedgingDesignationDomain_2" xlink:title="HedgingDesignationDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="HedgingDesignationAxis" xlink:to="HedgingDesignationDomain_2" xlink:title="definition: HedgingDesignationAxis to HedgingDesignationDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DesignatedAsHedgingInstrumentMember" xlink:label="DesignatedAsHedgingInstrumentMember" xlink:title="DesignatedAsHedgingInstrumentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="HedgingDesignationDomain" xlink:to="DesignatedAsHedgingInstrumentMember" xlink:title="definition: HedgingDesignationDomain to DesignatedAsHedgingInstrumentMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotDesignatedAsHedgingInstrumentEconomicHedgeMember" xlink:label="NotDesignatedAsHedgingInstrumentEconomicHedgeMember" xlink:title="NotDesignatedAsHedgingInstrumentEconomicHedgeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="HedgingDesignationDomain" xlink:to="NotDesignatedAsHedgingInstrumentEconomicHedgeMember" xlink:title="definition: HedgingDesignationDomain to NotDesignatedAsHedgingInstrumentEconomicHedgeMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="DerivativeInstrumentRiskAxis" xlink:title="DerivativeInstrumentRiskAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="DerivativeInstrumentRiskAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to DerivativeInstrumentRiskAxis" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="DerivativeContractTypeDomain" xlink:title="DerivativeContractTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="DerivativeInstrumentRiskAxis" xlink:to="DerivativeContractTypeDomain" xlink:title="definition: DerivativeInstrumentRiskAxis to DerivativeContractTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="DerivativeContractTypeDomain_2" xlink:title="DerivativeContractTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="DerivativeInstrumentRiskAxis" xlink:to="DerivativeContractTypeDomain_2" xlink:title="definition: DerivativeInstrumentRiskAxis to DerivativeContractTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignExchangeOptionMember" xlink:label="ForeignExchangeOptionMember" xlink:title="ForeignExchangeOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DerivativeContractTypeDomain" xlink:to="ForeignExchangeOptionMember" xlink:title="definition: DerivativeContractTypeDomain to ForeignExchangeOptionMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember" xlink:label="InterestRateSwapMember" xlink:title="InterestRateSwapMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DerivativeContractTypeDomain" xlink:to="InterestRateSwapMember" xlink:title="definition: DerivativeContractTypeDomain to InterestRateSwapMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/FinancialInstrumentsAndFairValueDisclosuresRealizedAndUnrealizedGainsAndLossesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:title="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnDerivativeInstrumentsNetPretaxAbstract" xlink:label="GainLossOnDerivativeInstrumentsNetPretaxAbstract" xlink:title="GainLossOnDerivativeInstrumentsNetPretaxAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="GainLossOnDerivativeInstrumentsNetPretaxAbstract" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to GainLossOnDerivativeInstrumentsNetPretaxAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfDerivatives" xlink:label="GainLossOnSaleOfDerivatives" xlink:title="GainLossOnSaleOfDerivatives" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="GainLossOnSaleOfDerivatives" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to GainLossOnSaleOfDerivatives" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrealizedGainLossOnDerivatives" xlink:label="UnrealizedGainLossOnDerivatives" xlink:title="UnrealizedGainLossOnDerivatives" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="UnrealizedGainLossOnDerivatives" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to UnrealizedGainLossOnDerivatives" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnDerivativeInstrumentsNetPretax" xlink:label="GainLossOnDerivativeInstrumentsNetPretax" xlink:title="GainLossOnDerivativeInstrumentsNetPretax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="GainLossOnDerivativeInstrumentsNetPretax" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to GainLossOnDerivativeInstrumentsNetPretax" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:title="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" order="5.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="DerivativeInstrumentRiskAxis" xlink:title="DerivativeInstrumentRiskAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="DerivativeInstrumentRiskAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to DerivativeInstrumentRiskAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="DerivativeContractTypeDomain" xlink:title="DerivativeContractTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="DerivativeInstrumentRiskAxis" xlink:to="DerivativeContractTypeDomain" xlink:title="definition: DerivativeInstrumentRiskAxis to DerivativeContractTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="DerivativeContractTypeDomain_2" xlink:title="DerivativeContractTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="DerivativeInstrumentRiskAxis" xlink:to="DerivativeContractTypeDomain_2" xlink:title="definition: DerivativeInstrumentRiskAxis to DerivativeContractTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignExchangeOptionMember" xlink:label="ForeignExchangeOptionMember" xlink:title="ForeignExchangeOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DerivativeContractTypeDomain" xlink:to="ForeignExchangeOptionMember" xlink:title="definition: DerivativeContractTypeDomain to ForeignExchangeOptionMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember" xlink:label="InterestRateSwapMember" xlink:title="InterestRateSwapMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DerivativeContractTypeDomain" xlink:to="InterestRateSwapMember" xlink:title="definition: DerivativeContractTypeDomain to InterestRateSwapMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="FairValueByFairValueHierarchyLevelAxis" xlink:title="FairValueByFairValueHierarchyLevelAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="FairValueByFairValueHierarchyLevelAxis" xlink:title="definition: FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable to FairValueByFairValueHierarchyLevelAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="FairValueMeasurementsFairValueHierarchyDomain" xlink:title="FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="FairValueByFairValueHierarchyLevelAxis" xlink:to="FairValueMeasurementsFairValueHierarchyDomain" xlink:title="definition: FairValueByFairValueHierarchyLevelAxis to FairValueMeasurementsFairValueHierarchyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="FairValueMeasurementsFairValueHierarchyDomain_2" xlink:title="FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="FairValueByFairValueHierarchyLevelAxis" xlink:to="FairValueMeasurementsFairValueHierarchyDomain_2" xlink:title="definition: FairValueByFairValueHierarchyLevelAxis to FairValueMeasurementsFairValueHierarchyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="FairValueInputsLevel2Member" xlink:title="FairValueInputsLevel2Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="FairValueMeasurementsFairValueHierarchyDomain" xlink:to="FairValueInputsLevel2Member" xlink:title="definition: FairValueMeasurementsFairValueHierarchyDomain to FairValueInputsLevel2Member" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LeasesLeaseCostDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LeasesOtherInformationDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LeasesUndiscountedCashFlowsOfLeaseLiabilityDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/LeasesUndiscountedCashFlowsOfLeaseLiabilityDetailsCalc01" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/CommitmentsAndContingenciesCommitmentsUnderLongtermLeaseContractsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesLineItems" xlink:label="LossContingenciesLineItems" xlink:title="LossContingenciesLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LessorOperatingLeasePaymentsFiscalYearMaturityAbstract" xlink:label="LessorOperatingLeasePaymentsFiscalYearMaturityAbstract" xlink:title="LessorOperatingLeasePaymentsFiscalYearMaturityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingenciesLineItems" xlink:to="LessorOperatingLeasePaymentsFiscalYearMaturityAbstract" xlink:title="definition: LossContingenciesLineItems to LessorOperatingLeasePaymentsFiscalYearMaturityAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LessorOperatingLeasePaymentsToBeReceivedNextRollingTwelveMonths" xlink:label="LessorOperatingLeasePaymentsToBeReceivedNextRollingTwelveMonths" xlink:title="LessorOperatingLeasePaymentsToBeReceivedNextRollingTwelveMonths" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingenciesLineItems" xlink:to="LessorOperatingLeasePaymentsToBeReceivedNextRollingTwelveMonths" xlink:title="definition: LossContingenciesLineItems to LessorOperatingLeasePaymentsToBeReceivedNextRollingTwelveMonths" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LessorOperatingLeasePaymentsToBeReceivedRollingYearTwo" xlink:label="LessorOperatingLeasePaymentsToBeReceivedRollingYearTwo" xlink:title="LessorOperatingLeasePaymentsToBeReceivedRollingYearTwo" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingenciesLineItems" xlink:to="LessorOperatingLeasePaymentsToBeReceivedRollingYearTwo" xlink:title="definition: LossContingenciesLineItems to LessorOperatingLeasePaymentsToBeReceivedRollingYearTwo" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LessorOperatingLeasePaymentsToBeReceived" xlink:label="LessorOperatingLeasePaymentsToBeReceived" xlink:title="LessorOperatingLeasePaymentsToBeReceived" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingenciesLineItems" xlink:to="LessorOperatingLeasePaymentsToBeReceived" xlink:title="definition: LossContingenciesLineItems to LessorOperatingLeasePaymentsToBeReceived" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitment" xlink:label="OtherCommitment" xlink:title="OtherCommitment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingenciesLineItems" xlink:to="OtherCommitment" xlink:title="definition: LossContingenciesLineItems to OtherCommitment" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesTable" xlink:label="LossContingenciesTable" xlink:title="LossContingenciesTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="LossContingenciesLineItems" xlink:to="LossContingenciesTable" xlink:title="definition: LossContingenciesLineItems to LossContingenciesTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:label="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:title="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="LossContingenciesTable" xlink:to="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:title="definition: LossContingenciesTable to IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:label="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:title="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:to="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:title="definition: IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis to DisposalGroupsIncludingDiscontinuedOperationsNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:label="DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2" xlink:title="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:to="DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2" xlink:title="definition: IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis to DisposalGroupsIncludingDiscontinuedOperationsNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" xlink:label="EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" xlink:title="EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:to="EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" xlink:title="definition: DisposalGroupsIncludingDiscontinuedOperationsNameDomain to EnergieparkHeringenPhilippsthalWPHPGmbHCoKGMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/CommitmentsAndContingenciesContingenciesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesLineItems" xlink:label="LossContingenciesLineItems" xlink:title="LossContingenciesLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyAbstract" xlink:label="LossContingencyAbstract" xlink:title="LossContingencyAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingenciesLineItems" xlink:to="LossContingencyAbstract" xlink:title="definition: LossContingenciesLineItems to LossContingencyAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyAccrualProvision" xlink:label="LossContingencyAccrualProvision" xlink:title="LossContingencyAccrualProvision" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingenciesLineItems" xlink:to="LossContingencyAccrualProvision" xlink:title="definition: LossContingenciesLineItems to LossContingencyAccrualProvision" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesTable" xlink:label="LossContingenciesTable" xlink:title="LossContingenciesTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="LossContingenciesLineItems" xlink:to="LossContingenciesTable" xlink:title="definition: LossContingenciesLineItems to LossContingenciesTable" order="3.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis" xlink:label="LossContingenciesByNatureOfContingencyAxis" xlink:title="LossContingenciesByNatureOfContingencyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="LossContingenciesTable" xlink:to="LossContingenciesByNatureOfContingencyAxis" xlink:title="definition: LossContingenciesTable to LossContingenciesByNatureOfContingencyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyNatureDomain" xlink:label="LossContingencyNatureDomain" xlink:title="LossContingencyNatureDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="LossContingenciesByNatureOfContingencyAxis" xlink:to="LossContingencyNatureDomain" xlink:title="definition: LossContingenciesByNatureOfContingencyAxis to LossContingencyNatureDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyNatureDomain" xlink:label="LossContingencyNatureDomain_2" xlink:title="LossContingencyNatureDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="LossContingenciesByNatureOfContingencyAxis" xlink:to="LossContingencyNatureDomain_2" xlink:title="definition: LossContingenciesByNatureOfContingencyAxis to LossContingencyNatureDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VariousCircumstancesInvolvingUncertaintyMember" xlink:label="VariousCircumstancesInvolvingUncertaintyMember" xlink:title="VariousCircumstancesInvolvingUncertaintyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingencyNatureDomain" xlink:to="VariousCircumstancesInvolvingUncertaintyMember" xlink:title="definition: LossContingencyNatureDomain to VariousCircumstancesInvolvingUncertaintyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PotentialProspectusErrorsForClosedEndFundsMember" xlink:label="PotentialProspectusErrorsForClosedEndFundsMember" xlink:title="PotentialProspectusErrorsForClosedEndFundsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LossContingencyNatureDomain" xlink:to="PotentialProspectusErrorsForClosedEndFundsMember" xlink:title="definition: LossContingencyNatureDomain to PotentialProspectusErrorsForClosedEndFundsMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/EarningsPerCommonShareDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDilutedLineItems" xlink:label="EarningsPerShareDilutedLineItems" xlink:title="EarningsPerShareDilutedLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDilutedOtherDisclosuresAbstract" xlink:label="EarningsPerShareDilutedOtherDisclosuresAbstract" xlink:title="EarningsPerShareDilutedOtherDisclosuresAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="EarningsPerShareDilutedOtherDisclosuresAbstract" xlink:title="definition: EarningsPerShareDilutedLineItems to EarningsPerShareDilutedOtherDisclosuresAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="ProfitLoss" xlink:title="ProfitLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="ProfitLoss" xlink:title="definition: EarningsPerShareDilutedLineItems to ProfitLoss" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:label="NetIncomeLossAttributableToNoncontrollingInterest" xlink:title="NetIncomeLossAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="NetIncomeLossAttributableToNoncontrollingInterest" xlink:title="definition: EarningsPerShareDilutedLineItems to NetIncomeLossAttributableToNoncontrollingInterest" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="NetIncomeLoss" xlink:title="NetIncomeLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="NetIncomeLoss" xlink:title="definition: EarningsPerShareDilutedLineItems to NetIncomeLoss" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RedeemablePreferredStockDividends" xlink:label="RedeemablePreferredStockDividends" xlink:title="RedeemablePreferredStockDividends" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="RedeemablePreferredStockDividends" xlink:title="definition: EarningsPerShareDilutedLineItems to RedeemablePreferredStockDividends" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:label="PreferredStockDividendsIncomeStatementImpact" xlink:title="PreferredStockDividendsIncomeStatementImpact" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="PreferredStockDividendsIncomeStatementImpact" xlink:title="definition: EarningsPerShareDilutedLineItems to PreferredStockDividendsIncomeStatementImpact" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="NetIncomeLossAvailableToCommonStockholdersBasic" xlink:title="NetIncomeLossAvailableToCommonStockholdersBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="NetIncomeLossAvailableToCommonStockholdersBasic" xlink:title="definition: EarningsPerShareDilutedLineItems to NetIncomeLossAvailableToCommonStockholdersBasic" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EffectOnSubsidiaryShareBasedExpensesDilutedEarningsPerShare" xlink:label="EffectOnSubsidiaryShareBasedExpensesDilutedEarningsPerShare" xlink:title="EffectOnSubsidiaryShareBasedExpensesDilutedEarningsPerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="EffectOnSubsidiaryShareBasedExpensesDilutedEarningsPerShare" xlink:title="definition: EarningsPerShareDilutedLineItems to EffectOnSubsidiaryShareBasedExpensesDilutedEarningsPerShare" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted" xlink:label="NetIncomeLossAvailableToCommonStockholdersDiluted" xlink:title="NetIncomeLossAvailableToCommonStockholdersDiluted" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="NetIncomeLossAvailableToCommonStockholdersDiluted" xlink:title="definition: EarningsPerShareDilutedLineItems to NetIncomeLossAvailableToCommonStockholdersDiluted" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="WeightedAverageNumberOfSharesOutstandingBasic" xlink:title="WeightedAverageNumberOfSharesOutstandingBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="WeightedAverageNumberOfSharesOutstandingBasic" xlink:title="definition: EarningsPerShareDilutedLineItems to WeightedAverageNumberOfSharesOutstandingBasic" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment" xlink:label="WeightedAverageNumberDilutedSharesOutstandingAdjustment" xlink:title="WeightedAverageNumberDilutedSharesOutstandingAdjustment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="WeightedAverageNumberDilutedSharesOutstandingAdjustment" xlink:title="definition: EarningsPerShareDilutedLineItems to WeightedAverageNumberDilutedSharesOutstandingAdjustment" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="WeightedAverageNumberOfDilutedSharesOutstanding" xlink:title="WeightedAverageNumberOfDilutedSharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="WeightedAverageNumberOfDilutedSharesOutstanding" xlink:title="definition: EarningsPerShareDilutedLineItems to WeightedAverageNumberOfDilutedSharesOutstanding" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic" xlink:label="EarningsPerShareBasic" xlink:title="EarningsPerShareBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="EarningsPerShareBasic" xlink:title="definition: EarningsPerShareDilutedLineItems to EarningsPerShareBasic" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="EarningsPerShareDiluted" xlink:title="EarningsPerShareDiluted" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="EarningsPerShareDiluted" xlink:title="definition: EarningsPerShareDilutedLineItems to EarningsPerShareDiluted" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable" xlink:label="ScheduleOfEarningsPerShareDilutedByCommonClassTable" xlink:title="ScheduleOfEarningsPerShareDilutedByCommonClassTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="EarningsPerShareDilutedLineItems" xlink:to="ScheduleOfEarningsPerShareDilutedByCommonClassTable" xlink:title="definition: EarningsPerShareDilutedLineItems to ScheduleOfEarningsPerShareDilutedByCommonClassTable" order="15.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="StatementClassOfStockAxis" xlink:title="StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfEarningsPerShareDilutedByCommonClassTable" xlink:to="StatementClassOfStockAxis" xlink:title="definition: ScheduleOfEarningsPerShareDilutedByCommonClassTable to StatementClassOfStockAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="ClassOfStockDomain_2" xlink:title="ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementClassOfStockAxis" xlink:to="ClassOfStockDomain_2" xlink:title="definition: StatementClassOfStockAxis to ClassOfStockDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesDPreferredStockMember" xlink:label="SeriesDPreferredStockMember" xlink:title="SeriesDPreferredStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ClassOfStockDomain" xlink:to="SeriesDPreferredStockMember" xlink:title="definition: ClassOfStockDomain to SeriesDPreferredStockMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/RevenuesVesselRevenuesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="DisaggregationOfRevenueLineItems" xlink:title="DisaggregationOfRevenueLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueAbstract" xlink:label="DisaggregationOfRevenueAbstract" xlink:title="DisaggregationOfRevenueAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DisaggregationOfRevenueLineItems" xlink:to="DisaggregationOfRevenueAbstract" xlink:title="definition: DisaggregationOfRevenueLineItems to DisaggregationOfRevenueAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLeaseIncome" xlink:label="OperatingLeaseLeaseIncome" xlink:title="OperatingLeaseLeaseIncome" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DisaggregationOfRevenueLineItems" xlink:to="OperatingLeaseLeaseIncome" xlink:title="definition: DisaggregationOfRevenueLineItems to OperatingLeaseLeaseIncome" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TimeCharterContractTerm" xlink:label="TimeCharterContractTerm" xlink:title="TimeCharterContractTerm" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DisaggregationOfRevenueLineItems" xlink:to="TimeCharterContractTerm" xlink:title="definition: DisaggregationOfRevenueLineItems to TimeCharterContractTerm" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="DisaggregationOfRevenueTable" xlink:title="DisaggregationOfRevenueTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="DisaggregationOfRevenueLineItems" xlink:to="DisaggregationOfRevenueTable" xlink:title="definition: DisaggregationOfRevenueLineItems to DisaggregationOfRevenueTable" order="4.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis" xlink:label="ProductOrServiceAxis" xlink:title="ProductOrServiceAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DisaggregationOfRevenueTable" xlink:to="ProductOrServiceAxis" xlink:title="definition: DisaggregationOfRevenueTable to ProductOrServiceAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="ProductsAndServicesDomain" xlink:title="ProductsAndServicesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ProductOrServiceAxis" xlink:to="ProductsAndServicesDomain" xlink:title="definition: ProductOrServiceAxis to ProductsAndServicesDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="ProductsAndServicesDomain_2" xlink:title="ProductsAndServicesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ProductOrServiceAxis" xlink:to="ProductsAndServicesDomain_2" xlink:title="definition: ProductOrServiceAxis to ProductsAndServicesDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TimeCharterMember" xlink:label="TimeCharterMember" xlink:title="TimeCharterMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ProductsAndServicesDomain" xlink:to="TimeCharterMember" xlink:title="definition: ProductsAndServicesDomain to TimeCharterMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_PoolAgreementsMember" xlink:label="PoolAgreementsMember" xlink:title="PoolAgreementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ProductsAndServicesDomain" xlink:to="PoolAgreementsMember" xlink:title="definition: ProductsAndServicesDomain to PoolAgreementsMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="RangeAxis" xlink:title="RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DisaggregationOfRevenueTable" xlink:to="RangeAxis" xlink:title="definition: DisaggregationOfRevenueTable to RangeAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="RangeMember" xlink:title="RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RangeAxis" xlink:to="RangeMember" xlink:title="definition: RangeAxis to RangeMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="RangeMember_2" xlink:title="RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RangeAxis" xlink:to="RangeMember_2" xlink:title="definition: RangeAxis to RangeMember_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="MinimumMember" xlink:title="MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RangeMember" xlink:to="MinimumMember" xlink:title="definition: RangeMember to MinimumMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="MaximumMember" xlink:title="MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RangeMember" xlink:to="MaximumMember" xlink:title="definition: RangeMember to MaximumMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/RevenuesRevenueFromServicesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="DisaggregationOfRevenueLineItems" xlink:title="DisaggregationOfRevenueLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueAbstract" xlink:label="DisaggregationOfRevenueAbstract" xlink:title="DisaggregationOfRevenueAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DisaggregationOfRevenueLineItems" xlink:to="DisaggregationOfRevenueAbstract" xlink:title="definition: DisaggregationOfRevenueLineItems to DisaggregationOfRevenueAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DisaggregationOfRevenueLineItems" xlink:to="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="definition: DisaggregationOfRevenueLineItems to RevenueFromContractWithCustomerExcludingAssessedTax" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="DisaggregationOfRevenueTable" xlink:title="DisaggregationOfRevenueTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="DisaggregationOfRevenueLineItems" xlink:to="DisaggregationOfRevenueTable" xlink:title="definition: DisaggregationOfRevenueLineItems to DisaggregationOfRevenueTable" order="3.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis" xlink:label="ProductOrServiceAxis" xlink:title="ProductOrServiceAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DisaggregationOfRevenueTable" xlink:to="ProductOrServiceAxis" xlink:title="definition: DisaggregationOfRevenueTable to ProductOrServiceAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="ProductsAndServicesDomain" xlink:title="ProductsAndServicesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ProductOrServiceAxis" xlink:to="ProductsAndServicesDomain" xlink:title="definition: ProductOrServiceAxis to ProductsAndServicesDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="ProductsAndServicesDomain_2" xlink:title="ProductsAndServicesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ProductOrServiceAxis" xlink:to="ProductsAndServicesDomain_2" xlink:title="definition: ProductOrServiceAxis to ProductsAndServicesDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ShipManagementMember" xlink:label="ShipManagementMember" xlink:title="ShipManagementMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ProductsAndServicesDomain" xlink:to="ShipManagementMember" xlink:title="definition: ProductsAndServicesDomain to ShipManagementMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ManagementServiceMember" xlink:label="ManagementServiceMember" xlink:title="ManagementServiceMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ProductsAndServicesDomain" xlink:to="ManagementServiceMember" xlink:title="definition: ProductsAndServicesDomain to ManagementServiceMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TransactionServicesMember" xlink:label="TransactionServicesMember" xlink:title="TransactionServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ProductsAndServicesDomain" xlink:to="TransactionServicesMember" xlink:title="definition: ProductsAndServicesDomain to TransactionServicesMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_OtherRevenueMember" xlink:label="OtherRevenueMember" xlink:title="OtherRevenueMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ProductsAndServicesDomain" xlink:to="OtherRevenueMember" xlink:title="definition: ProductsAndServicesDomain to OtherRevenueMember" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="StatementGeographicalAxis" xlink:title="StatementGeographicalAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="DisaggregationOfRevenueTable" xlink:to="StatementGeographicalAxis" xlink:title="definition: DisaggregationOfRevenueTable to StatementGeographicalAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="SegmentGeographicalDomain" xlink:title="SegmentGeographicalDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementGeographicalAxis" xlink:to="SegmentGeographicalDomain" xlink:title="definition: StatementGeographicalAxis to SegmentGeographicalDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="SegmentGeographicalDomain_2" xlink:title="SegmentGeographicalDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementGeographicalAxis" xlink:to="SegmentGeographicalDomain_2" xlink:title="definition: StatementGeographicalAxis to SegmentGeographicalDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_DE" xlink:label="DE" xlink:title="DE" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="DE" xlink:title="definition: SegmentGeographicalDomain to DE" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_NL" xlink:label="NL" xlink:title="NL" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="NL" xlink:title="definition: SegmentGeographicalDomain to NL" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CN" xlink:label="CN" xlink:title="CN" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="CN" xlink:title="definition: SegmentGeographicalDomain to CN" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_SG" xlink:label="SG" xlink:title="SG" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="SG" xlink:title="definition: SegmentGeographicalDomain to SG" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_PA" xlink:label="PA" xlink:title="PA" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="PA" xlink:title="definition: SegmentGeographicalDomain to PA" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CO" xlink:label="CO" xlink:title="CO" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentGeographicalDomain" xlink:to="CO" xlink:title="definition: SegmentGeographicalDomain to CO" order="6.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/VesselOperatingExpensesAndVoyageExpensesDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/GeneralAndAdministrativeExpensesDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/CostOfRevenueFromServicesDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/InterestAndFinanceCostsDetails">
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InterestAndFinanceCostsLineItems" xlink:label="InterestAndFinanceCostsLineItems" xlink:title="InterestAndFinanceCostsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseAbstract" xlink:label="InterestExpenseAbstract" xlink:title="InterestExpenseAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InterestAndFinanceCostsLineItems" xlink:to="InterestExpenseAbstract" xlink:title="definition: InterestAndFinanceCostsLineItems to InterestExpenseAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt" xlink:label="InterestExpenseDebt" xlink:title="InterestExpenseDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InterestAndFinanceCostsLineItems" xlink:to="InterestExpenseDebt" xlink:title="definition: InterestAndFinanceCostsLineItems to InterestExpenseDebt" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_AmortizationOfDebtIssuanceCostsAndWriteOffOfDeferredDebtIssuanceCosts" xlink:label="AmortizationOfDebtIssuanceCostsAndWriteOffOfDeferredDebtIssuanceCosts" xlink:title="AmortizationOfDebtIssuanceCostsAndWriteOffOfDeferredDebtIssuanceCosts" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InterestAndFinanceCostsLineItems" xlink:to="AmortizationOfDebtIssuanceCostsAndWriteOffOfDeferredDebtIssuanceCosts" xlink:title="definition: InterestAndFinanceCostsLineItems to AmortizationOfDebtIssuanceCostsAndWriteOffOfDeferredDebtIssuanceCosts" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseOther" xlink:label="InterestExpenseOther" xlink:title="InterestExpenseOther" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InterestAndFinanceCostsLineItems" xlink:to="InterestExpenseOther" xlink:title="definition: InterestAndFinanceCostsLineItems to InterestExpenseOther" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="InterestExpenseNonoperating" xlink:title="InterestExpenseNonoperating" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InterestAndFinanceCostsLineItems" xlink:to="InterestExpenseNonoperating" xlink:title="definition: InterestAndFinanceCostsLineItems to InterestExpenseNonoperating" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_InterestAndFinanceCostsTable" xlink:label="InterestAndFinanceCostsTable" xlink:title="InterestAndFinanceCostsTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="InterestAndFinanceCostsLineItems" xlink:to="InterestAndFinanceCostsTable" xlink:title="definition: InterestAndFinanceCostsLineItems to InterestAndFinanceCostsTable" order="6.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="InterestAndFinanceCostsTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: InterestAndFinanceCostsTable to RelatedPartyTransactionsByRelatedPartyAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="NonrelatedPartyMember" xlink:title="NonrelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="NonrelatedPartyMember" xlink:title="definition: RelatedPartyDomain to NonrelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IncomeTaxesProvisionForIncomeTaxesExpenseOrBenefitDetails">
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_IncomeTaxDisclosureLineItems" xlink:label="IncomeTaxDisclosureLineItems" xlink:title="IncomeTaxDisclosureLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" xlink:label="IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" xlink:title="IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" xlink:title="definition: IncomeTaxDisclosureLineItems to IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComponentsOfIncomeTaxExpenseBenefitContinuingOperationsAbstract" xlink:label="ComponentsOfIncomeTaxExpenseBenefitContinuingOperationsAbstract" xlink:title="ComponentsOfIncomeTaxExpenseBenefitContinuingOperationsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="ComponentsOfIncomeTaxExpenseBenefitContinuingOperationsAbstract" xlink:title="definition: IncomeTaxDisclosureLineItems to ComponentsOfIncomeTaxExpenseBenefitContinuingOperationsAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract" xlink:label="CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract" xlink:title="CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract" xlink:title="definition: IncomeTaxDisclosureLineItems to CurrentIncomeTaxExpenseBenefitContinuingOperationsAbstract" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CurrentFederalTaxExpenseBenefit" xlink:label="CurrentFederalTaxExpenseBenefit" xlink:title="CurrentFederalTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="CurrentFederalTaxExpenseBenefit" xlink:title="definition: IncomeTaxDisclosureLineItems to CurrentFederalTaxExpenseBenefit" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CurrentStateAndLocalTaxExpenseBenefit" xlink:label="CurrentStateAndLocalTaxExpenseBenefit" xlink:title="CurrentStateAndLocalTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="CurrentStateAndLocalTaxExpenseBenefit" xlink:title="definition: IncomeTaxDisclosureLineItems to CurrentStateAndLocalTaxExpenseBenefit" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CurrentForeignTaxExpenseBenefit" xlink:label="CurrentForeignTaxExpenseBenefit" xlink:title="CurrentForeignTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="CurrentForeignTaxExpenseBenefit" xlink:title="definition: IncomeTaxDisclosureLineItems to CurrentForeignTaxExpenseBenefit" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherTaxExpenseBenefit" xlink:label="OtherTaxExpenseBenefit" xlink:title="OtherTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="OtherTaxExpenseBenefit" xlink:title="definition: IncomeTaxDisclosureLineItems to OtherTaxExpenseBenefit" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="IncomeTaxExpenseBenefit" xlink:title="IncomeTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="IncomeTaxExpenseBenefit" xlink:title="definition: IncomeTaxDisclosureLineItems to IncomeTaxExpenseBenefit" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CurrentIncomeTaxExpenseBenefit" xlink:label="CurrentIncomeTaxExpenseBenefit" xlink:title="CurrentIncomeTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="CurrentIncomeTaxExpenseBenefit" xlink:title="definition: IncomeTaxDisclosureLineItems to CurrentIncomeTaxExpenseBenefit" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit" xlink:label="DeferredIncomeTaxExpenseBenefit" xlink:title="DeferredIncomeTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="DeferredIncomeTaxExpenseBenefit" xlink:title="definition: IncomeTaxDisclosureLineItems to DeferredIncomeTaxExpenseBenefit" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_CorporateIncomeTax" xlink:label="CorporateIncomeTax" xlink:title="CorporateIncomeTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="CorporateIncomeTax" xlink:title="definition: IncomeTaxDisclosureLineItems to CorporateIncomeTax" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_TradeTaxesPayable" xlink:label="TradeTaxesPayable" xlink:title="TradeTaxesPayable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="TradeTaxesPayable" xlink:title="definition: IncomeTaxDisclosureLineItems to TradeTaxesPayable" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_OtherTaxesPayable" xlink:label="OtherTaxesPayable" xlink:title="OtherTaxesPayable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="OtherTaxesPayable" xlink:title="definition: IncomeTaxDisclosureLineItems to OtherTaxesPayable" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedIncomeTaxesCurrent" xlink:label="AccruedIncomeTaxesCurrent" xlink:title="AccruedIncomeTaxesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="AccruedIncomeTaxesCurrent" xlink:title="definition: IncomeTaxDisclosureLineItems to AccruedIncomeTaxesCurrent" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesReceivable" xlink:label="IncomeTaxesReceivable" xlink:title="IncomeTaxesReceivable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="IncomeTaxesReceivable" xlink:title="definition: IncomeTaxDisclosureLineItems to IncomeTaxesReceivable" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_IncomeTaxDisclosureTable" xlink:label="IncomeTaxDisclosureTable" xlink:title="IncomeTaxDisclosureTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="IncomeTaxDisclosureLineItems" xlink:to="IncomeTaxDisclosureTable" xlink:title="definition: IncomeTaxDisclosureLineItems to IncomeTaxDisclosureTable" order="16.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="BusinessAcquisitionAxis" xlink:title="BusinessAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="IncomeTaxDisclosureTable" xlink:to="BusinessAcquisitionAxis" xlink:title="definition: IncomeTaxDisclosureTable to BusinessAcquisitionAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="BusinessAcquisitionAcquireeDomain" xlink:title="BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="BusinessAcquisitionAxis" xlink:to="BusinessAcquisitionAcquireeDomain" xlink:title="definition: BusinessAcquisitionAxis to BusinessAcquisitionAcquireeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="BusinessAcquisitionAcquireeDomain_2" xlink:title="BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="BusinessAcquisitionAxis" xlink:to="BusinessAcquisitionAcquireeDomain_2" xlink:title="definition: BusinessAcquisitionAxis to BusinessAcquisitionAcquireeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCMunchmeyerPetersenCapitalAGMember" xlink:label="MPCMunchmeyerPetersenCapitalAGMember" xlink:title="MPCMunchmeyerPetersenCapitalAGMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="BusinessAcquisitionAcquireeDomain" xlink:to="MPCMunchmeyerPetersenCapitalAGMember" xlink:title="definition: BusinessAcquisitionAcquireeDomain to MPCMunchmeyerPetersenCapitalAGMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IncomeTaxesEffectiveIncomeTaxRateReconciliationDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationLineItems" xlink:label="EffectiveIncomeTaxRateReconciliationLineItems" xlink:title="EffectiveIncomeTaxRateReconciliationLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract" xlink:label="EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract" xlink:title="EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" xlink:label="IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" xlink:title="IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:label="IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:title="IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:label="IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:title="IncomeTaxReconciliationStateAndLocalIncomeTaxes" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxReconciliationStateAndLocalIncomeTaxes" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential" xlink:label="IncomeTaxReconciliationForeignIncomeTaxRateDifferential" xlink:title="IncomeTaxReconciliationForeignIncomeTaxRateDifferential" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxReconciliationForeignIncomeTaxRateDifferential" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxReconciliationForeignIncomeTaxRateDifferential" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:label="IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:title="IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpense" xlink:label="IncomeTaxReconciliationNondeductibleExpense" xlink:title="IncomeTaxReconciliationNondeductibleExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxReconciliationNondeductibleExpense" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxReconciliationNondeductibleExpense" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationPriorYearIncomeTaxes" xlink:label="IncomeTaxReconciliationPriorYearIncomeTaxes" xlink:title="IncomeTaxReconciliationPriorYearIncomeTaxes" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxReconciliationPriorYearIncomeTaxes" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxReconciliationPriorYearIncomeTaxes" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationOtherReconcilingItems" xlink:label="IncomeTaxReconciliationOtherReconcilingItems" xlink:title="IncomeTaxReconciliationOtherReconcilingItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxReconciliationOtherReconcilingItems" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxReconciliationOtherReconcilingItems" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="IncomeTaxExpenseBenefit" xlink:title="IncomeTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="IncomeTaxExpenseBenefit" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to IncomeTaxExpenseBenefit" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:title="EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:label="EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:title="EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential" xlink:label="EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential" xlink:title="EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:label="EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:title="EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpense" xlink:label="EffectiveIncomeTaxRateReconciliationNondeductibleExpense" xlink:title="EffectiveIncomeTaxRateReconciliationNondeductibleExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateReconciliationNondeductibleExpense" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateReconciliationNondeductibleExpense" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes" xlink:label="EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes" xlink:title="EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes" order="16.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent" xlink:label="EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent" xlink:title="EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent" order="17.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations" xlink:label="EffectiveIncomeTaxRateContinuingOperations" xlink:title="EffectiveIncomeTaxRateContinuingOperations" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateContinuingOperations" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateContinuingOperations" order="18.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="EffectiveIncomeTaxRateReconciliationTable" xlink:title="EffectiveIncomeTaxRateReconciliationTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="EffectiveIncomeTaxRateReconciliationLineItems" xlink:to="EffectiveIncomeTaxRateReconciliationTable" xlink:title="definition: EffectiveIncomeTaxRateReconciliationLineItems to EffectiveIncomeTaxRateReconciliationTable" order="19.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityNameAxis" xlink:label="IncomeTaxAuthorityNameAxis" xlink:title="IncomeTaxAuthorityNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="EffectiveIncomeTaxRateReconciliationTable" xlink:to="IncomeTaxAuthorityNameAxis" xlink:title="definition: EffectiveIncomeTaxRateReconciliationTable to IncomeTaxAuthorityNameAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityNameDomain" xlink:label="IncomeTaxAuthorityNameDomain" xlink:title="IncomeTaxAuthorityNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="IncomeTaxAuthorityNameAxis" xlink:to="IncomeTaxAuthorityNameDomain" xlink:title="definition: IncomeTaxAuthorityNameAxis to IncomeTaxAuthorityNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityNameDomain" xlink:label="IncomeTaxAuthorityNameDomain_2" xlink:title="IncomeTaxAuthorityNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="IncomeTaxAuthorityNameAxis" xlink:to="IncomeTaxAuthorityNameDomain_2" xlink:title="definition: IncomeTaxAuthorityNameAxis to IncomeTaxAuthorityNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalMinistryOfFinanceGermanyMember" xlink:label="FederalMinistryOfFinanceGermanyMember" xlink:title="FederalMinistryOfFinanceGermanyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IncomeTaxAuthorityNameDomain" xlink:to="FederalMinistryOfFinanceGermanyMember" xlink:title="definition: IncomeTaxAuthorityNameDomain to FederalMinistryOfFinanceGermanyMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IncomeTaxesProvisionForIncomeTaxesExpenseOrBenefitDetailsCalc2" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IncomeTaxesComponentsOfDeferredTaxAccountBalancesDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/IncomeTaxesUncertainTaxPositionsDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SharebasedCompensationAssumptionsForOptionsGrantedDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:title="SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:title="ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" order="9.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="AwardTypeAxis" xlink:title="AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="AwardTypeAxis" xlink:title="definition: ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable to AwardTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:title="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="AwardTypeAxis" xlink:to="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:title="definition: AwardTypeAxis to ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2" xlink:title="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="AwardTypeAxis" xlink:to="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2" xlink:title="definition: AwardTypeAxis to ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="EmployeeStockOptionMember" xlink:title="EmployeeStockOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="EmployeeStockOptionMember" xlink:title="definition: ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain to EmployeeStockOptionMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SharebasedCompensationOptionsActivityDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedAbstract" xlink:label="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedAbstract" xlink:title="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedAbstract" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedAbstract" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:label="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:title="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:title="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="AllocatedShareBasedCompensationExpense" xlink:title="AllocatedShareBasedCompensationExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="AllocatedShareBasedCompensationExpense" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to AllocatedShareBasedCompensationExpense" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:label="PaymentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:title="PaymentsRelatedToTaxWithholdingForShareBasedCompensation" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="PaymentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to PaymentsRelatedToTaxWithholdingForShareBasedCompensation" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:title="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:title="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:title="ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:title="ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" order="16.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" order="17.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" order="18.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:title="StockIssuedDuringPeriodSharesStockOptionsExercised" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to StockIssuedDuringPeriodSharesStockOptionsExercised" order="19.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" order="20.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:title="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" order="21.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:title="ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:title="definition: ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems to ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" order="22.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="AwardTypeAxis" xlink:title="AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="AwardTypeAxis" xlink:title="definition: ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable to AwardTypeAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:title="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="AwardTypeAxis" xlink:to="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:title="definition: AwardTypeAxis to ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2" xlink:title="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="AwardTypeAxis" xlink:to="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2" xlink:title="definition: AwardTypeAxis to ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="EmployeeStockOptionMember" xlink:title="EmployeeStockOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="EmployeeStockOptionMember" xlink:title="definition: ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain to EmployeeStockOptionMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SegmentInformationInformationAboutCompanySReportableSegmentsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="SegmentReportingInformationLineItems" xlink:title="SegmentReportingInformationLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationProfitLossAbstract" xlink:label="SegmentReportingInformationProfitLossAbstract" xlink:title="SegmentReportingInformationProfitLossAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="SegmentReportingInformationProfitLossAbstract" xlink:title="definition: SegmentReportingInformationLineItems to SegmentReportingInformationProfitLossAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLeaseIncome" xlink:label="OperatingLeaseLeaseIncome" xlink:title="OperatingLeaseLeaseIncome" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="OperatingLeaseLeaseIncome" xlink:title="definition: SegmentReportingInformationLineItems to OperatingLeaseLeaseIncome" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="RevenueFromContractWithCustomerExcludingAssessedTax" xlink:title="definition: SegmentReportingInformationLineItems to RevenueFromContractWithCustomerExcludingAssessedTax" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Revenues" xlink:label="Revenues" xlink:title="Revenues" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="Revenues" xlink:title="definition: SegmentReportingInformationLineItems to Revenues" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_VoyageExpenses" xlink:label="VoyageExpenses" xlink:title="VoyageExpenses" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="VoyageExpenses" xlink:title="definition: SegmentReportingInformationLineItems to VoyageExpenses" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingCostsAndExpenses" xlink:label="OperatingCostsAndExpenses" xlink:title="OperatingCostsAndExpenses" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="OperatingCostsAndExpenses" xlink:title="definition: SegmentReportingInformationLineItems to OperatingCostsAndExpenses" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:label="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:title="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:title="definition: SegmentReportingInformationLineItems to CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_RelatedPartyTransactionManagementFees" xlink:label="RelatedPartyTransactionManagementFees" xlink:title="RelatedPartyTransactionManagementFees" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="RelatedPartyTransactionManagementFees" xlink:title="definition: SegmentReportingInformationLineItems to RelatedPartyTransactionManagementFees" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="DepreciationDepletionAndAmortization" xlink:title="DepreciationDepletionAndAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="DepreciationDepletionAndAmortization" xlink:title="definition: SegmentReportingInformationLineItems to DepreciationDepletionAndAmortization" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForDoubtfulAccounts" xlink:label="ProvisionForDoubtfulAccounts" xlink:title="ProvisionForDoubtfulAccounts" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="ProvisionForDoubtfulAccounts" xlink:title="definition: SegmentReportingInformationLineItems to ProvisionForDoubtfulAccounts" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="GeneralAndAdministrativeExpense" xlink:title="GeneralAndAdministrativeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="GeneralAndAdministrativeExpense" xlink:title="definition: SegmentReportingInformationLineItems to GeneralAndAdministrativeExpense" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnDispositionOfAssets" xlink:label="GainLossOnDispositionOfAssets" xlink:title="GainLossOnDispositionOfAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="GainLossOnDispositionOfAssets" xlink:title="definition: SegmentReportingInformationLineItems to GainLossOnDispositionOfAssets" order="12.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:label="ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:title="ImpairmentOfLongLivedAssetsToBeDisposedOf" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="ImpairmentOfLongLivedAssetsToBeDisposedOf" xlink:title="definition: SegmentReportingInformationLineItems to ImpairmentOfLongLivedAssetsToBeDisposedOf" order="13.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeconsolidationGainOrLossAmount" xlink:label="DeconsolidationGainOrLossAmount" xlink:title="DeconsolidationGainOrLossAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="DeconsolidationGainOrLossAmount" xlink:title="definition: SegmentReportingInformationLineItems to DeconsolidationGainOrLossAmount" order="14.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:label="EquitySecuritiesFvNiUnrealizedGainLoss" xlink:title="EquitySecuritiesFvNiUnrealizedGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="EquitySecuritiesFvNiUnrealizedGainLoss" xlink:title="definition: SegmentReportingInformationLineItems to EquitySecuritiesFvNiUnrealizedGainLoss" order="15.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:label="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:title="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" xlink:title="definition: SegmentReportingInformationLineItems to EquityMethodInvestmentsMeasuredAtFairValueFVNIUnrealizedGainLoss" order="16.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss" xlink:label="OperatingIncomeLoss" xlink:title="OperatingIncomeLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="OperatingIncomeLoss" xlink:title="definition: SegmentReportingInformationLineItems to OperatingIncomeLoss" order="17.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="InterestExpenseNonoperating" xlink:title="InterestExpenseNonoperating" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="InterestExpenseNonoperating" xlink:title="definition: SegmentReportingInformationLineItems to InterestExpenseNonoperating" order="18.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInterest" xlink:label="InvestmentIncomeInterest" xlink:title="InvestmentIncomeInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="InvestmentIncomeInterest" xlink:title="definition: SegmentReportingInformationLineItems to InvestmentIncomeInterest" order="19.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" xlink:label="ForeignCurrencyTransactionGainLossBeforeTax" xlink:title="ForeignCurrencyTransactionGainLossBeforeTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="ForeignCurrencyTransactionGainLossBeforeTax" xlink:title="definition: SegmentReportingInformationLineItems to ForeignCurrencyTransactionGainLossBeforeTax" order="20.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromEquityMethodInvestments" xlink:label="IncomeLossFromEquityMethodInvestments" xlink:title="IncomeLossFromEquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="IncomeLossFromEquityMethodInvestments" xlink:title="definition: SegmentReportingInformationLineItems to IncomeLossFromEquityMethodInvestments" order="21.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeDividend" xlink:label="InvestmentIncomeDividend" xlink:title="InvestmentIncomeDividend" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="InvestmentIncomeDividend" xlink:title="definition: SegmentReportingInformationLineItems to InvestmentIncomeDividend" order="22.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" xlink:label="EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" xlink:title="EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" xlink:title="definition: SegmentReportingInformationLineItems to EquityMethodInvestmentsMeasuredAtFairValueDividendIncome" order="23.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiGainLoss" xlink:label="EquitySecuritiesFvNiGainLoss" xlink:title="EquitySecuritiesFvNiGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="EquitySecuritiesFvNiGainLoss" xlink:title="definition: SegmentReportingInformationLineItems to EquitySecuritiesFvNiGainLoss" order="24.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesTradingGainLoss" xlink:label="DebtSecuritiesTradingGainLoss" xlink:title="DebtSecuritiesTradingGainLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="DebtSecuritiesTradingGainLoss" xlink:title="definition: SegmentReportingInformationLineItems to DebtSecuritiesTradingGainLoss" order="25.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="OtherNonoperatingIncomeExpense" xlink:title="OtherNonoperatingIncomeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="OtherNonoperatingIncomeExpense" xlink:title="definition: SegmentReportingInformationLineItems to OtherNonoperatingIncomeExpense" order="26.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:label="IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:title="IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingInformationLineItems" xlink:to="IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:title="definition: SegmentReportingInformationLineItems to IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" order="27.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="ScheduleOfSegmentReportingInformationBySegmentTable" xlink:title="ScheduleOfSegmentReportingInformationBySegmentTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="SegmentReportingInformationLineItems" xlink:to="ScheduleOfSegmentReportingInformationBySegmentTable" xlink:title="definition: SegmentReportingInformationLineItems to ScheduleOfSegmentReportingInformationBySegmentTable" order="28.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsAxis" xlink:label="ConsolidationItemsAxis" xlink:title="ConsolidationItemsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="ConsolidationItemsAxis" xlink:title="definition: ScheduleOfSegmentReportingInformationBySegmentTable to ConsolidationItemsAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain" xlink:label="ConsolidationItemsDomain" xlink:title="ConsolidationItemsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ConsolidationItemsAxis" xlink:to="ConsolidationItemsDomain" xlink:title="definition: ConsolidationItemsAxis to ConsolidationItemsDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain" xlink:label="ConsolidationItemsDomain_2" xlink:title="ConsolidationItemsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ConsolidationItemsAxis" xlink:to="ConsolidationItemsDomain_2" xlink:title="definition: ConsolidationItemsAxis to ConsolidationItemsDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingSegmentsMember" xlink:label="OperatingSegmentsMember" xlink:title="OperatingSegmentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ConsolidationItemsDomain" xlink:to="OperatingSegmentsMember" xlink:title="definition: ConsolidationItemsDomain to OperatingSegmentsMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MaterialReconcilingItemsMember" xlink:label="MaterialReconcilingItemsMember" xlink:title="MaterialReconcilingItemsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ConsolidationItemsDomain" xlink:to="MaterialReconcilingItemsMember" xlink:title="definition: ConsolidationItemsDomain to MaterialReconcilingItemsMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateNonSegmentMember" xlink:label="CorporateNonSegmentMember" xlink:title="CorporateNonSegmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ConsolidationItemsDomain" xlink:to="CorporateNonSegmentMember" xlink:title="definition: ConsolidationItemsDomain to CorporateNonSegmentMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="StatementBusinessSegmentsAxis" xlink:title="StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="StatementBusinessSegmentsAxis" xlink:title="definition: ScheduleOfSegmentReportingInformationBySegmentTable to StatementBusinessSegmentsAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="SegmentDomain" xlink:title="SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementBusinessSegmentsAxis" xlink:to="SegmentDomain" xlink:title="definition: StatementBusinessSegmentsAxis to SegmentDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="SegmentDomain_2" xlink:title="SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementBusinessSegmentsAxis" xlink:to="SegmentDomain_2" xlink:title="definition: StatementBusinessSegmentsAxis to SegmentDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DryBulkSegmentMember" xlink:label="DryBulkSegmentMember" xlink:title="DryBulkSegmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentDomain" xlink:to="DryBulkSegmentMember" xlink:title="definition: SegmentDomain to DryBulkSegmentMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ContainershipSegmentMember" xlink:label="ContainershipSegmentMember" xlink:title="ContainershipSegmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentDomain" xlink:to="ContainershipSegmentMember" xlink:title="definition: SegmentDomain to ContainershipSegmentMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_AssetManagementSegmentMember" xlink:label="AssetManagementSegmentMember" xlink:title="AssetManagementSegmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentDomain" xlink:to="AssetManagementSegmentMember" xlink:title="definition: SegmentDomain to AssetManagementSegmentMember" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="definition: ScheduleOfSegmentReportingInformationBySegmentTable to RelatedPartyTransactionsByRelatedPartyAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="RelatedPartyDomain_2" xlink:title="RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="RelatedPartyDomain_2" xlink:title="definition: RelatedPartyTransactionsByRelatedPartyAxis to RelatedPartyDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="NonrelatedPartyMember" xlink:title="NonrelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="NonrelatedPartyMember" xlink:title="definition: RelatedPartyDomain to NonrelatedPartyMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="RelatedPartyMember" xlink:title="RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="RelatedPartyDomain" xlink:to="RelatedPartyMember" xlink:title="definition: RelatedPartyDomain to RelatedPartyMember" order="2.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SegmentInformationReconciliationOfTotalSegmentAssetsToTotalAssetsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingAssetReconcilingItemLineItems" xlink:label="SegmentReportingAssetReconcilingItemLineItems" xlink:title="SegmentReportingAssetReconcilingItemLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReconciliationFromSegmentTotalsToConsolidatedAbstract" xlink:label="ReconciliationFromSegmentTotalsToConsolidatedAbstract" xlink:title="ReconciliationFromSegmentTotalsToConsolidatedAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingAssetReconcilingItemLineItems" xlink:to="ReconciliationFromSegmentTotalsToConsolidatedAbstract" xlink:title="definition: SegmentReportingAssetReconcilingItemLineItems to ReconciliationFromSegmentTotalsToConsolidatedAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="Assets" xlink:title="Assets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingAssetReconcilingItemLineItems" xlink:to="Assets" xlink:title="definition: SegmentReportingAssetReconcilingItemLineItems to Assets" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="CashAndCashEquivalentsAtCarryingValue" xlink:title="CashAndCashEquivalentsAtCarryingValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingAssetReconcilingItemLineItems" xlink:to="CashAndCashEquivalentsAtCarryingValue" xlink:title="definition: SegmentReportingAssetReconcilingItemLineItems to CashAndCashEquivalentsAtCarryingValue" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssets" xlink:label="PrepaidExpenseAndOtherAssets" xlink:title="PrepaidExpenseAndOtherAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentReportingAssetReconcilingItemLineItems" xlink:to="PrepaidExpenseAndOtherAssets" xlink:title="definition: SegmentReportingAssetReconcilingItemLineItems to PrepaidExpenseAndOtherAssets" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReconciliationOfAssetsFromSegmentToConsolidatedTable" xlink:label="ReconciliationOfAssetsFromSegmentToConsolidatedTable" xlink:title="ReconciliationOfAssetsFromSegmentToConsolidatedTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="SegmentReportingAssetReconcilingItemLineItems" xlink:to="ReconciliationOfAssetsFromSegmentToConsolidatedTable" xlink:title="definition: SegmentReportingAssetReconcilingItemLineItems to ReconciliationOfAssetsFromSegmentToConsolidatedTable" order="5.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsAxis" xlink:label="ConsolidationItemsAxis" xlink:title="ConsolidationItemsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ReconciliationOfAssetsFromSegmentToConsolidatedTable" xlink:to="ConsolidationItemsAxis" xlink:title="definition: ReconciliationOfAssetsFromSegmentToConsolidatedTable to ConsolidationItemsAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain" xlink:label="ConsolidationItemsDomain" xlink:title="ConsolidationItemsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ConsolidationItemsAxis" xlink:to="ConsolidationItemsDomain" xlink:title="definition: ConsolidationItemsAxis to ConsolidationItemsDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain" xlink:label="ConsolidationItemsDomain_2" xlink:title="ConsolidationItemsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ConsolidationItemsAxis" xlink:to="ConsolidationItemsDomain_2" xlink:title="definition: ConsolidationItemsAxis to ConsolidationItemsDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingSegmentsMember" xlink:label="OperatingSegmentsMember" xlink:title="OperatingSegmentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ConsolidationItemsDomain" xlink:to="OperatingSegmentsMember" xlink:title="definition: ConsolidationItemsDomain to OperatingSegmentsMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MaterialReconcilingItemsMember" xlink:label="MaterialReconcilingItemsMember" xlink:title="MaterialReconcilingItemsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ConsolidationItemsDomain" xlink:to="MaterialReconcilingItemsMember" xlink:title="definition: ConsolidationItemsDomain to MaterialReconcilingItemsMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="StatementBusinessSegmentsAxis" xlink:title="StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="ReconciliationOfAssetsFromSegmentToConsolidatedTable" xlink:to="StatementBusinessSegmentsAxis" xlink:title="definition: ReconciliationOfAssetsFromSegmentToConsolidatedTable to StatementBusinessSegmentsAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="SegmentDomain" xlink:title="SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementBusinessSegmentsAxis" xlink:to="SegmentDomain" xlink:title="definition: StatementBusinessSegmentsAxis to SegmentDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="SegmentDomain_2" xlink:title="SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementBusinessSegmentsAxis" xlink:to="SegmentDomain_2" xlink:title="definition: StatementBusinessSegmentsAxis to SegmentDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_DryBulkSegmentMember" xlink:label="DryBulkSegmentMember" xlink:title="DryBulkSegmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentDomain" xlink:to="DryBulkSegmentMember" xlink:title="definition: SegmentDomain to DryBulkSegmentMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_ContainershipSegmentMember" xlink:label="ContainershipSegmentMember" xlink:title="ContainershipSegmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentDomain" xlink:to="ContainershipSegmentMember" xlink:title="definition: SegmentDomain to ContainershipSegmentMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_AssetManagementSegmentMember" xlink:label="AssetManagementSegmentMember" xlink:title="AssetManagementSegmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SegmentDomain" xlink:to="AssetManagementSegmentMember" xlink:title="definition: SegmentDomain to AssetManagementSegmentMember" order="3.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://castormaritime.com/role/SubsequentEventsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems" xlink:label="SubsequentEventLineItems" xlink:title="SubsequentEventLineItems" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_SubsequentEventAbstract" xlink:label="SubsequentEventAbstract" xlink:title="SubsequentEventAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="SubsequentEventAbstract" xlink:title="definition: SubsequentEventLineItems to SubsequentEventAbstract" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_JointVentureEquityInterestPercentage" xlink:label="JointVentureEquityInterestPercentage" xlink:title="JointVentureEquityInterestPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="JointVentureEquityInterestPercentage" xlink:title="definition: SubsequentEventLineItems to JointVentureEquityInterestPercentage" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_CashConsiderationReceivedOnContributionOfVesselToJointVenture" xlink:label="CashConsiderationReceivedOnContributionOfVesselToJointVenture" xlink:title="CashConsiderationReceivedOnContributionOfVesselToJointVenture" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="CashConsiderationReceivedOnContributionOfVesselToJointVenture" xlink:title="definition: SubsequentEventLineItems to CashConsiderationReceivedOnContributionOfVesselToJointVenture" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="DebtInstrumentFaceAmount" xlink:title="DebtInstrumentFaceAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="DebtInstrumentFaceAmount" xlink:title="definition: SubsequentEventLineItems to DebtInstrumentFaceAmount" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:label="SaleOfStockNumberOfSharesIssuedInTransaction" xlink:title="SaleOfStockNumberOfSharesIssuedInTransaction" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="SaleOfStockNumberOfSharesIssuedInTransaction" xlink:title="definition: SubsequentEventLineItems to SaleOfStockNumberOfSharesIssuedInTransaction" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="SaleOfStockPricePerShare" xlink:title="SaleOfStockPricePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="SaleOfStockPricePerShare" xlink:title="definition: SubsequentEventLineItems to SaleOfStockPricePerShare" order="6.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockConsiderationReceivedPerTransaction" xlink:label="SaleOfStockConsiderationReceivedPerTransaction" xlink:title="SaleOfStockConsiderationReceivedPerTransaction" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="SaleOfStockConsiderationReceivedPerTransaction" xlink:title="definition: SubsequentEventLineItems to SaleOfStockConsiderationReceivedPerTransaction" order="7.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction" xlink:label="SaleOfStockPercentageOfOwnershipAfterTransaction" xlink:title="SaleOfStockPercentageOfOwnershipAfterTransaction" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="SaleOfStockPercentageOfOwnershipAfterTransaction" xlink:title="definition: SubsequentEventLineItems to SaleOfStockPercentageOfOwnershipAfterTransaction" order="8.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="EquityMethodInvestmentOwnershipPercentage" xlink:title="EquityMethodInvestmentOwnershipPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="EquityMethodInvestmentOwnershipPercentage" xlink:title="definition: SubsequentEventLineItems to EquityMethodInvestmentOwnershipPercentage" order="9.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_JointVentureGainsLossesNet" xlink:label="JointVentureGainsLossesNet" xlink:title="JointVentureGainsLossesNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventLineItems" xlink:to="JointVentureGainsLossesNet" xlink:title="definition: SubsequentEventLineItems to JointVentureGainsLossesNet" order="10.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable" xlink:label="SubsequentEventTable" xlink:title="SubsequentEventTable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="SubsequentEventLineItems" xlink:to="SubsequentEventTable" xlink:title="definition: SubsequentEventLineItems to SubsequentEventTable" order="11.0" t1:closed="true" t1:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis" xlink:label="StatementScenarioAxis" xlink:title="StatementScenarioAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="SubsequentEventTable" xlink:to="StatementScenarioAxis" xlink:title="definition: SubsequentEventTable to StatementScenarioAxis" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="ScenarioUnspecifiedDomain" xlink:title="ScenarioUnspecifiedDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="StatementScenarioAxis" xlink:to="ScenarioUnspecifiedDomain" xlink:title="definition: StatementScenarioAxis to ScenarioUnspecifiedDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="ScenarioUnspecifiedDomain_2" xlink:title="ScenarioUnspecifiedDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="StatementScenarioAxis" xlink:to="ScenarioUnspecifiedDomain_2" xlink:title="definition: StatementScenarioAxis to ScenarioUnspecifiedDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioForecastMember" xlink:label="ScenarioForecastMember" xlink:title="ScenarioForecastMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ScenarioUnspecifiedDomain" xlink:to="ScenarioForecastMember" xlink:title="definition: ScenarioUnspecifiedDomain to ScenarioForecastMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="SubsequentEventTypeAxis" xlink:title="SubsequentEventTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="SubsequentEventTable" xlink:to="SubsequentEventTypeAxis" xlink:title="definition: SubsequentEventTable to SubsequentEventTypeAxis" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="SubsequentEventTypeDomain" xlink:title="SubsequentEventTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="SubsequentEventTypeAxis" xlink:to="SubsequentEventTypeDomain" xlink:title="definition: SubsequentEventTypeAxis to SubsequentEventTypeDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="SubsequentEventTypeDomain_2" xlink:title="SubsequentEventTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="SubsequentEventTypeAxis" xlink:to="SubsequentEventTypeDomain_2" xlink:title="definition: SubsequentEventTypeAxis to SubsequentEventTypeDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember" xlink:label="SubsequentEventMember" xlink:title="SubsequentEventMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SubsequentEventTypeDomain" xlink:to="SubsequentEventMember" xlink:title="definition: SubsequentEventTypeDomain to SubsequentEventMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="DebtInstrumentAxis" xlink:title="DebtInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="SubsequentEventTable" xlink:to="DebtInstrumentAxis" xlink:title="definition: SubsequentEventTable to DebtInstrumentAxis" order="3.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="DebtInstrumentNameDomain_2" xlink:title="DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="DebtInstrumentAxis" xlink:to="DebtInstrumentNameDomain_2" xlink:title="definition: DebtInstrumentAxis to DebtInstrumentNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_SeniorTermLoanFacilityMember" xlink:label="SeniorTermLoanFacilityMember" xlink:title="SeniorTermLoanFacilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="DebtInstrumentNameDomain" xlink:to="SeniorTermLoanFacilityMember" xlink:title="definition: DebtInstrumentNameDomain to SeniorTermLoanFacilityMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="SubsequentEventTable" xlink:to="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:title="definition: SubsequentEventTable to ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="4.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="EquityMethodInvesteeNameDomain_2" xlink:title="EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="EquityMethodInvesteeNameDomain_2" xlink:title="definition: ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis to EquityMethodInvesteeNameDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="ctrm-20260630.xsd#ctrm_MPCContainerShipsASAMember" xlink:label="MPCContainerShipsASAMember" xlink:title="MPCContainerShipsASAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="EquityMethodInvesteeNameDomain" xlink:to="MPCContainerShipsASAMember" xlink:title="definition: EquityMethodInvesteeNameDomain to MPCContainerShipsASAMember" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="SubsidiarySaleOfStockAxis" xlink:title="SubsidiarySaleOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="SubsequentEventTable" xlink:to="SubsidiarySaleOfStockAxis" xlink:title="definition: SubsequentEventTable to SubsidiarySaleOfStockAxis" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="SaleOfStockNameOfTransactionDomain" xlink:title="SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="SubsidiarySaleOfStockAxis" xlink:to="SaleOfStockNameOfTransactionDomain" xlink:title="definition: SubsidiarySaleOfStockAxis to SaleOfStockNameOfTransactionDomain" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="SaleOfStockNameOfTransactionDomain_2" xlink:title="SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="SubsidiarySaleOfStockAxis" xlink:to="SaleOfStockNameOfTransactionDomain_2" xlink:title="definition: SubsidiarySaleOfStockAxis to SaleOfStockNameOfTransactionDomain_2" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrivatePlacementMember" xlink:label="PrivatePlacementMember" xlink:title="PrivatePlacementMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="SaleOfStockNameOfTransactionDomain" xlink:to="PrivatePlacementMember" xlink:title="definition: SaleOfStockNameOfTransactionDomain to PrivatePlacementMember" order="1.0" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" />
</link:linkbase>
