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

    <documentType>4</documentType><periodOfReport>2004-01-16</periodOfReport>
    <issuer>
        <issuerCik>0000097854</issuerCik>
        <issuerName>THOMAS &amp; BETTS CORP</issuerName>
        <issuerTradingSymbol>TNB</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001198352</rptOwnerCik>
            <rptOwnerName>DUNNIGAN T KEVIN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>8155 T&amp;B BOULEVARD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MEMPHIS</rptOwnerCity>
            <rptOwnerState>TN</rptOwnerState>
            <rptOwnerZipCode>38125</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>2004-01-16</value></transactionDate><deemedExecutionDate><value>2004-01-16</value></deemedExecutionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>40061</value><footnoteId id="F1"/></transactionShares><transactionPricePerShare><value>23.42</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>265114</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><derivativeTable><derivativeTransaction><securityTitle><value>Common Stock</value></securityTitle><conversionOrExercisePrice><value>23.42</value></conversionOrExercisePrice><transactionDate><value>2004-01-16</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>93476</value><footnoteId id="F2"/></transactionShares><transactionPricePerShare><value>23.42</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>2004-01-16</value></exerciseDate><expirationDate><value>2014-01-16</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Employee Stock Option (right to buy)</value></underlyingSecurityTitle><underlyingSecurityShares><value>93476</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>93476</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction></derivativeTable><footnotes><footnote id="F1">Stock award granted pursuant to T. Kevin Dunnigan's Retirement Agreement dated December 2, 2003.  </footnote><footnote id="F2">Stock Option award granted pursuant to T. Kevin Dunnigan's Retirement Agreement dated December 2, 2003.  Options are fully vested and exercisable on the date the options were granted.</footnote></footnotes><remarks/><ownerSignature><signatureName>Penelope Y. Turnbow</signatureName><signatureDate>2004-01-16</signatureDate></ownerSignature></ownershipDocument>

