<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-07-01</periodOfReport><notSubjectToSection16>1</notSubjectToSection16>
    <issuer>
        <issuerCik>0000930429</issuerCik>
        <issuerName>MONTEREY BAY BANCORP INC</issuerName>
        <issuerTradingSymbol>MBBC</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001186121</rptOwnerCik>
            <rptOwnerName>JAMES JONI</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>SVP, Director of I.T. </officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeSecurity><securityTitle><value>Common Stock </value></securityTitle><transactionDate><value>2003-07-01</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F1"/></transactionCoding><transactionAmounts><transactionShares><value>845.69</value></transactionShares><transactionValue><value>26.3245</value></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>0</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By ESOP</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><derivativeSecurity><securityTitle><value>Stock Option</value></securityTitle><conversionOrExercisePrice><value>16.26</value></conversionOrExercisePrice><transactionDate><value>2003-07-01</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F2"/></transactionCoding><transactionAmounts><transactionShares><value>3000</value></transactionShares><transactionValue><value>26.3245</value><footnoteId id="F2"/></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>1988-08-08</value><footnoteId id="F2"/></exerciseDate><expirationDate><value>2011-09-10</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>3000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>0</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><derivativeSecurity><securityTitle><value>Stock Option</value></securityTitle><conversionOrExercisePrice><value>16.82</value></conversionOrExercisePrice><transactionDate><value>2003-07-01</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F2"/></transactionCoding><transactionAmounts><transactionShares><value>2000</value></transactionShares><transactionValue><value>26.3245</value><footnoteId id="F2"/></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>1988-08-08</value><footnoteId id="F2"/></exerciseDate><expirationDate><value>2011-12-20</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>2000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>0</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">Cash out of shares held in Employee Stock Ownership Plan pursuant to the Agreement and  Plan of Merger by and among Union Bank of California, N.A., UnionBanCal Corporation, Monterey Bay Bank and Monterey Bay Bancorp, Inc. dated April 7, 2003. </footnote><footnote id="F2">These options, which provided for vesting in 5 installments beginning September 10, 2002 and December 20, 2002, respectively, were cancelled in the merger in exchange for a cash payment representing the difference between approximately $26.3245 and the exercise price of each such cancelled option. </footnote></footnotes><ownerSignature><signatureName>Joni James</signatureName><signatureDate>2003-07-01</signatureDate></ownerSignature></ownershipDocument>

