<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-12-03</periodOfReport>
    <issuer>
        <issuerCik>0000874501</issuerCik>
        <issuerName>AMBAC FINANCIAL GROUP INC</issuerName>
        <issuerTradingSymbol>ABK</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001194129</rptOwnerCik>
            <rptOwnerName>CONSIDINE JILL M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>AMBAC FINANCIAL GROUP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>ONE STATE STREET PLAZA</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10004</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-12-03</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5</value><footnoteId id="F1"/></transactionShares><transactionPricePerShare><value>69.44</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>3082</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><derivativeTable><derivativeTransaction><securityTitle><value>Phantom Stock Units</value></securityTitle><conversionOrExercisePrice><footnoteId id="F6"/></conversionOrExercisePrice><transactionDate><value>2003-12-03</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>4</value></transactionShares><transactionPricePerShare><value>69.44</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><footnoteId id="F2"/></exerciseDate><expirationDate><footnoteId id="F3"/></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>4</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>2347</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction><derivativeTransaction><securityTitle><value>Phantom Stock Units</value></securityTitle><conversionOrExercisePrice><footnoteId id="F7"/></conversionOrExercisePrice><transactionDate><value>2003-12-03</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>162</value></transactionShares><transactionPricePerShare><value>69.44</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><footnoteId id="F4"/></exerciseDate><expirationDate><footnoteId id="F5"/></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>162</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>2509</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction></derivativeTable><footnotes><footnote id="F1">Represents dividends on Restricted Stock Units (&quot;RSUs&quot;) granted to the Reporting Person under the Issuer's 1997 Non-Employee Directors Equity Plan, as amended, which dividends are paid in the form of additional RSUs.</footnote><footnote id="F2">Represents dividends on Phantom Stock Units acquired under the Issuer's Deferred Compensation Plan and are to be settled 100% in cash upon the Reporting Person's retirement or resignation from the Issuer's Board of Directors or on an earlier dated previously selected by the Reporting Person.</footnote><footnote id="F3">Represents dividends on Phantom Stock Units acquired under the Issuer's Deferred Compensation Plan and are to be settled 100% in cash upon the Reporting Person's retirement or resignation from the Issuer's Board of Directors or on an earlier dated previously selected by the Reporting Person.</footnote><footnote id="F4">These Phantom Stock Units (&quot;PSU&quot;) were acquired under the Issuer's Deferred Compensation Plan and represent 25% of the Reporting Person's Annual Fees for serving as a director of a committee and 100% of her Meeting fees earned during the third quarter of 2003.  These PSUs are to be settled 100% in cash upon the Reporting Person's retirement or resignation from the Issuer's Board of Directors or on an earlier date previously selected by the Reporting Person.</footnote><footnote id="F5">These Phantom Stock Units (&quot;PSU) were acquired under the Issuer's Deferred Compensation Plan and represent 25% of the Reporting Person's Annual Fees for serving as a director of a committee and 100% of her Meeting fees earned during the third quarter of 2003.  These PSUs are to be settled 100% in cash upon the Reporting Person's retirement or resignation from the Issuer's Board of Directors or on an earlier date previously selected by the Reporting Person.</footnote><footnote id="F6">Each Phantom Stock Unit converts to common stock on a one to one basis.</footnote><footnote id="F7">Each Phantom Stock Unit converts to common stock on a one to one basis.</footnote></footnotes><remarks/><ownerSignature><signatureName>Anne G. Gill, Attorney-in-fact for Jill M. Considine</signatureName><signatureDate>2003-12-04</signatureDate></ownerSignature></ownershipDocument>

