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

    <documentType>4</documentType><periodOfReport>2003-10-31</periodOfReport>
    <issuer>
        <issuerCik>0001236964</issuerCik>
        <issuerName>KNBT BANCORP INC</issuerName>
        <issuerTradingSymbol>KNBT</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001247013</rptOwnerCik>
            <rptOwnerName>HUGHES DAVID W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O KEYSTONE NAZARETH BANK &amp; TRUST CO.</rptOwnerStreet1>
            <rptOwnerStreet2>90 HIGHLAND AVENUE</rptOwnerStreet2>
            <rptOwnerCity>BETHLEHEM</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>18017</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>See Footnote (6)</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-10-31</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>658</value></transactionShares><transactionPricePerShare><footnoteId id="F1"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>658</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By ESOP </value><footnoteId id="F2"/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><derivativeTable><derivativeTransaction><securityTitle><value>Employee Stock Option (right to buy)</value></securityTitle><conversionOrExercisePrice><value>4.2703</value></conversionOrExercisePrice><transactionDate><value>2003-10-31</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>10197</value></transactionShares><transactionPricePerShare><footnoteId id="F3"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><footnoteId id="F3"/></exerciseDate><expirationDate><value>2011-06-21</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>10197</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>10197</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction><derivativeTransaction><securityTitle><value>Employee Stock Option (right to buy)</value></securityTitle><conversionOrExercisePrice><value>5.7786</value></conversionOrExercisePrice><transactionDate><value>2003-10-31</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>9713</value></transactionShares><transactionPricePerShare><footnoteId id="F4"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><footnoteId id="F4"/></exerciseDate><expirationDate><value>2012-01-17</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>9713</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>9713</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction><derivativeTransaction><securityTitle><value>Employee Stock Option (right to buy)</value></securityTitle><conversionOrExercisePrice><value>6.4405</value></conversionOrExercisePrice><transactionDate><value>2003-10-31</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>11100</value></transactionShares><transactionPricePerShare><footnoteId id="F5"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><footnoteId id="F5"/></exerciseDate><expirationDate><value>2013-01-16</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>11100</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>11100</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction></derivativeTable><footnotes><footnote id="F1">Received in exchange for 178 shares of First Colonial Group, Inc. (&quot;First Colonial&quot;) common stock in connection with the merger of First Colonial into KNBT Bancorp, Inc. (the &quot;Merger&quot;).  On the effective date of the Merger, the closing price of First Colonial's common stock was $63.35 per share and market value of the Issuer's common stock was $10.00 per share.</footnote><footnote id="F2">The shares are held in the First Colonial employee stock ownership plan. Such plan will be terminated and the shares held thereby distributed to participants upon receipt of a qualified determination letter.</footnote><footnote id="F3">Received in the Merger in exchange for a stock option to acquire 2,756 shares of First Colonial common stock for $15.8000 per share which became exercisable as a result of First Colonial entering into the merger agreement in March 2003.</footnote><footnote id="F4">Received in the Merger in exchange for a stock option to acquire 2,625 shares of First Colonial common stock for $21.3810 per share which became exercisable as a result of First Colonial entering into the merger agreement in March 2003.</footnote><footnote id="F5">Received in the Merger in exchange for a stock option to acquire 3,000 shares of First Colonial common stock for $23.8300 per share which became exercisable as a result of First Colonial entering into the merger agreement in March 2003.</footnote></footnotes><remarks>(6) - Senior Vice President, Marketing, Public Relations and Community Development of Keystone Nazareth Bank &amp; Trust Company (Issuer Subsidiary).</remarks><ownerSignature><signatureName>David W. Hughes</signatureName><signatureDate>2003-11-04</signatureDate></ownerSignature></ownershipDocument>

