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

    <documentType>4</documentType><periodOfReport>2003-06-30</periodOfReport>
    <issuer>
        <issuerCik>0001008554</issuerCik>
        <issuerName>HUMBOLDT BANCORP</issuerName>
        <issuerTradingSymbol>HBEK</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001102033</rptOwnerCik>
            <rptOwnerName>EVANS GARY L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><derivativeSecurity><securityTitle><value>Deferred Rights</value></securityTitle><conversionOrExercisePrice><value>0</value><footnoteId id="F2"/></conversionOrExercisePrice><transactionDate><value>2003-06-30</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>204</value><footnoteId id="F1"/></transactionShares><transactionValue><value>14.94</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>1988-08-08</value><footnoteId id="F2"/></exerciseDate><expirationDate><value>1988-08-08</value><footnoteId id="F2"/></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>204</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>3688</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">Adjusted for Cash Dividend March 24, 03, $0.025/share (6 shares issued) &amp;amp; the Cash Dividend June 12, 03, $0.030/share (7 shares issued). The net of 191 are the shares purhcased through the deferral plan.</footnote><footnote id="F2">Represents the reporting person's director's fee under Humboldt Bancorp's deferred compensation plan. Under the plan, the director has the right to defer their director's fee to be paid in common stock upon resignation or retirement. The conversion price of the derivative security is 1 for 1, the number of shares purchased for the month are based on the closing price on the last trade date of the month for which the director fees are paid. </footnote></footnotes><ownerSignature><signatureName>Gary L. Evans</signatureName><signatureDate>2003-07-01</signatureDate></ownerSignature><ownerSignature><signatureName>Signed by Patrick J. Rusnak, Power of Attorney on File</signatureName><signatureDate>2003-07-01</signatureDate></ownerSignature></ownershipDocument>

