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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2013-06-25</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000872912</issuerCik>
        <issuerName>DELCATH SYSTEMS, INC.</issuerName>
        <issuerTradingSymbol>DCTH</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001288536</rptOwnerCik>
            <rptOwnerName>APPLING WILLIAM M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O DELCATH SYSTEMS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>566 QUEENSBURY AVENUE</rptOwnerStreet2>
            <rptOwnerCity>QUEENSBURY</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>12804</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP, R&amp;D and Global Operations</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>common stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10250</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>common stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>common stock</value>
                <footnoteId id="F2"/>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4200</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>common stock</value>
                <footnoteId id="F3"/>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>600</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by member of household</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>common stock</value>
                <footnoteId id="F4"/>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>525</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by member of household</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>stock option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>8.14</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-08-09</value>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2020-08-09</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>60000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>stock option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>6.42</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-03-10</value>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <value>2021-03-10</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>18000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>stock option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.60</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2013-02-28</value>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <value>2022-02-28</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>25200</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>stock option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.13</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2014-03-11</value>
                <footnoteId id="F8"/>
            </exerciseDate>
            <expirationDate>
                <value>2023-03-11</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>25200</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>stock option</value>
                <footnoteId id="F9"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.12</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-08-15</value>
            </exerciseDate>
            <expirationDate>
                <value>2021-08-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2750</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by member of household</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>stock option</value>
                <footnoteId id="F10"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.60</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2013-02-28</value>
            </exerciseDate>
            <expirationDate>
                <value>2022-02-28</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2100</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by member of household</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>stock option</value>
                <footnoteId id="F11"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.13</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2014-03-11</value>
            </exerciseDate>
            <expirationDate>
                <value>2023-03-11</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2100</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by member of household</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The restricted stock vested ratably on March 10, 2012 and 2013, and the remainder will vest ratably on March 11, 2014, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.</footnote>
        <footnote id="F2">The restricted stock vested ratably on February 28, 2013 and will continue to vest ratably on February 28, 2014 and 2015, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.</footnote>
        <footnote id="F3">The restricted stock vested ratably on August 15, 2012 and will continue to vest ratably on August 15, 2013 and 2014, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.  The reporting person disclaims beneficial ownership of these securities, and this report shall not be deemed an admission that the reporting person is the beneficial owner of such securities for purposes of Section 16 or any other purpose.</footnote>
        <footnote id="F4">The restricted stock vested ratably on February 28, 2013 and will continue to vest ratably on February 28, 2014 and 2015, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.  The reporting person disclaims beneficial ownership of these securities, and this report shall not be deemed an admission that the reporting person is the beneficial owner of such securities for purposes of Section 16 or any other purpose.</footnote>
        <footnote id="F5">The stock option vested ratably on August 9, 2011 and August 9, 2012 and  the remainder will vest on August 9, 2013, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.</footnote>
        <footnote id="F6">The stock option vested ratably on March 10, 2012, and March 10, 2013 and the remainder will vest on March 10, 2014, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.</footnote>
        <footnote id="F7">The stock option vested ratably on February 28, 2013, and will continue to vest ratably on February 28, 2014 and 2015, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.</footnote>
        <footnote id="F8">The stock option vests ratably on March 11, 2014, 2015 and 2016, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.</footnote>
        <footnote id="F9">The stock option vested ratably on August 15, 2012 and will continue to vest ratably on August 15, 2013 and 2014, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events.  The reporting person disclaims beneficial ownership of these securities, and this report shall not be deemed an admisison that the reporting person is the beneficial owner of such securities for purposes of Section 16 or for any other purpose.</footnote>
        <footnote id="F10">The stock option vested ratably on February 28, 2013 and will continue to vest ratably on February 28, 2014 and 2015, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events. The reporting person disclaims beneficial ownership of these securities, and this report shall not be deemed an admisison that the reporting person is the beneficial owner of such securities for purposes of Section 16 or for any other purpose.</footnote>
        <footnote id="F11">The stock option will vest ratably on March 11, 2014, 2015 and 2016, and is subject to forfeiture in the event of certain circumstances and acceleration upon certain events. The reporting person disclaims beneficial ownership of these securities, and this report shall not be deemed an admisison that the reporting person is the beneficial owner of such securities for purposes of Section 16 or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Barbra Keck, pursuant to a confirming statement executed by William Appling</signatureName>
        <signatureDate>2013-07-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
