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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2014-03-03</periodOfReport>

    <dateOfOriginalSubmission>2014-03-05</dateOfOriginalSubmission>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001177648</issuerCik>
        <issuerName>ENANTA PHARMACEUTICALS INC</issuerName>
        <issuerTradingSymbol>ENTA</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001183073</rptOwnerCik>
            <rptOwnerName>TVM V LIFE SCIENCE VENTURES GMBH &amp; CO KG</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL CROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001342938</rptOwnerCik>
            <rptOwnerName>TVM V Life Science Ventures Management GmbH &amp; Co. KG</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001106929</rptOwnerCik>
            <rptOwnerName>TVM IV GMBH &amp; CO KG</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001342940</rptOwnerCik>
            <rptOwnerName>TVM IV Management GmbH &amp; Co. KG</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001133935</rptOwnerCik>
            <rptOwnerName>TVM MEDICAL VENTURES GMBH &amp; CO KG</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001342939</rptOwnerCik>
            <rptOwnerName>TVM Capital GmbH</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001398598</rptOwnerCik>
            <rptOwnerName>Birner Hubert</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001398596</rptOwnerCik>
            <rptOwnerName>Fischer Stefan</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001397372</rptOwnerCik>
            <rptOwnerName>Goll Alexandra</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001133921</rptOwnerCik>
            <rptOwnerName>SCHUHSLER HELMUT</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TVM CAPITAL GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>OTTOSTRASSE 4</rptOwnerStreet2>
            <rptOwnerCity>MUNICH</rptOwnerCity>
            <rptOwnerState>2M</rptOwnerState>
            <rptOwnerZipCode>80333</rptOwnerZipCode>
            <rptOwnerStateDescription>GERMANY</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-03-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1402</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>38.8337</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>343505</value>
                    <footnoteId id="F6"/>
                </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>2014-03-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1874</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>38.8337</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>459064</value>
                    <footnoteId id="F6"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-03-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5795</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>38.8337</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1419812</value>
                    <footnoteId id="F6"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-03-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>66506</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>38.8155</value>
                    <footnoteId id="F5"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>276999</value>
                    <footnoteId id="F6"/>
                </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>2014-03-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>88877</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>38.8155</value>
                    <footnoteId id="F5"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>370187</value>
                    <footnoteId id="F6"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-03-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>274880</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>38.8155</value>
                    <footnoteId id="F5"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1144932</value>
                    <footnoteId id="F6"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $38.80 to $39.03, inclusive. Each Reporting Person undertakes to provide to Enanta Pharmaceuticals, Inc. (the &quot;Issuer&quot;), any security holder of the Issuer, or the staff of the Securities and Exchange Commission (&quot;SEC&quot;), upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F2">The shares are directly held by TVM Medical Ventures GmbH &amp; Co. KG (&quot;Medical Ventures&quot;), the general partner of which is TVM Capital GmbH (&quot;TVM Capital&quot;), for which Alexandra Goll (&quot;Goll&quot;) and Helmut Schuhsler (&quot;Schuhsler&quot;), each members of the investment committee of TVM Capital, share voting and investment authority over the shares held by Medical Ventures with the other members of the investment committee. Each of TVM Capital, Goll and Schuhsler disclaims beneficial ownership of these shares except to the extent of their pecuniary interest therein, if any.</footnote>
        <footnote id="F3">The shares are directly held by TVM IV GmbH &amp; Co. KG (&quot;TVM IV&quot;), the managing limited partner of which is TVM IV Management GmbH &amp; Co. KG (&quot;TVM IV Management&quot;), for which Goll and Schuhsler, each members of the investment committee of TVM IV Management, share voting and investment authority over the shares held by TVM IV with the other members of the investment committee. Each of TVM IV Management, Goll and Schuhsler disclaims beneficial ownership of these shares except to the extent of their pecuniary interest therein, if any.</footnote>
        <footnote id="F4">The shares are directly held by TVM V Life Science Ventures GmbH &amp; Co. KG (&quot;TVM V&quot;), the managing limited partner of which is TVM V Life Science Ventures Management GmbH &amp; Co. KG (&quot;TVM V Management&quot;), for which Hubert Birner (&quot;Birner&quot;), Stefan Fischer (&quot;Fischer&quot;), Goll and Schuhsler, each members of the investment committee of TVM V Management, share voting and investment authority over the shares held by TVM V. Each of TVM V Management, Birner, Fischer, Goll and Schuhsler disclaims beneficial ownership of these shares except to the extent of their pecuniary interest therein, if any.</footnote>
        <footnote id="F5">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $38.80 to $39.62, inclusive. Each Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F6">The totals in Column 5 are being amended in order to correct a calculation error with respect to the number of shares held by each Reporting Person after the transactions reported herein and on the Form 4 filed with the SEC by each Reporting Person on 2/27/2014 (the &quot;Previous Filing&quot;).  The number of shares disposed of by each Reporting Person and reported herein and in the Previous Filing were accurately reported and have not been amended.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Josef Moosholzer, Managing Limited Partner of TVM V Life Science Ventures Management GmbH &amp; Co. KG, managing limited partner of TVM V Life Science Ventures GmbH &amp; Co. KG</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Josef Moosholzer, Managing Limited Partner of TVM V Life Science Ventures Management GmbH &amp; Co. KG</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Josef Moosholzer, Managing Limited Partner of TVM IV Mangement GmbH &amp; Co. KG, managing limited partner of TVM IV GmbH &amp; Co. KG</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Josef Moosholzer, Managing Limited Partner of TVM IV Mangement GmbH &amp; Co. KG</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Josef Moosholzer, Managing Limited Partner of TVM Capital GmbH, general partner of TVM Medical Ventures GmbH &amp; Co. KG</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Josef Moosholzer, Managing Limited Partner of TVM Capital GmbH</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Rolf Starck by power of Attorney for Hubert Birner</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Rolf Starck by power of Attorney for Stefan Fischer</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Rolf Starck by power of Attorney for Alexandra Goll</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Rolf Starck by power of Attorney for Helmut Schuhsler</signatureName>
        <signatureDate>2014-03-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
