<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-09-05</periodOfReport>

    <issuer>
        <issuerCik>0001293282</issuerCik>
        <issuerName>TechTarget Inc</issuerName>
        <issuerTradingSymbol>TTGT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001082906</rptOwnerCik>
            <rptOwnerName>HOAG JAY C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001197937</rptOwnerCik>
            <rptOwnerName>KIMBALL RICK</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001188456</rptOwnerCik>
            <rptOwnerName>DREW JOHN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001188444</rptOwnerCik>
            <rptOwnerName>REYNOLDS JON Q JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001188458</rptOwnerCik>
            <rptOwnerName>GRIFFITH WILLIAM</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001211137</rptOwnerCik>
            <rptOwnerName>FEINBERG HENRY</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001283643</rptOwnerCik>
            <rptOwnerName>TECHNOLOGY CROSSOVER MANAGEMENT V LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001274664</rptOwnerCik>
            <rptOwnerName>TCV V LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001283642</rptOwnerCik>
            <rptOwnerName>TCV MEMBER FUND L P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TECHNOLOGY CROSSOVER VENTURES</rptOwnerStreet1>
            <rptOwnerStreet2>528 RAMONA STREET</rptOwnerStreet2>
            <rptOwnerCity>PALO ALTO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94301</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>May be part of 13(g) group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2008-09-05</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>758</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2008-09-05</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>758</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3735</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>TCV Management 2004, L.L.C.</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12150808</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>TCV V, L.P.</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>231106</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>TCV Member Fund, L.P.</value>
                    <footnoteId id="F5"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Jay C. Hoag (&quot;Hoag&quot;) transferred 758 shares which were issued under the TechTarget, Inc. 2007 Stock Option and Incentive Plan to TCV Management 2004, L.L.C. (&quot;Management&quot;). While Hoag was the beneficial owner of these shares, Management was entitled to 100% of the pecuniary interest in such shares prior to their transfer.</footnote>
        <footnote id="F2">These shares were directly held by Hoag.  Hoag had sole dispositive and voting power over the shares; however, Management owned 100% of the pecuniary interest therein.  Hoag is a member of Management but disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F3">These shares are direcly held by Management. Hoag is a member of Management but disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F4">These shares are directly held by TCV V, L.P.  Each of Hoag, Richard H. Kimball, John L. Drew,  Jon Q. Reynolds, Jr., William J.G. Griffith IV and Henry J. Feinberg (collectively the &quot;TCM Members&quot;) are Class A Members of Technology Crossover Management V, L.L.C. (&quot;TCM V&quot;) which is the general partner of TCV V, L.P.  The TCM Members and TCM V may be deemed to beneficially own the shares held by TCV V, L.P. but the TCM Members and TCM V disclaim beneficial ownership of such shares except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F5">These shares are directly held by TCV Member Fund, L.P.  Each of the TCM Members are (i) Class A Members of TCM V which is a general partner of TCV Member Fund, L.P. and (ii) limited partners of TCV Member Fund, L.P.  The TCM Members and TCM V may be deemed to beneficially own the shares held by TCV Member Fund, L.P. but the TCM Members and TCM V disclaim beneficial ownership of such shares except to the extent of their pecuniary interest therein.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for Jay C. Hoag</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for Richard H. Kimball</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for John L. Drew</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for Jon Q. Reynolds Jr.</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for William J.G. Griffith IV</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for Henry J. Feinberg</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for Technology Crossover Management V, L.L.C.</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for TCV V, L.P.</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carla S. Newell  Authorized signatory for TCV Member Fund, L.P.</signatureName>
        <signatureDate>2008-09-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
