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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-08-29</periodOfReport>

    <issuer>
        <issuerCik>0000097216</issuerCik>
        <issuerName>TEREX CORP</issuerName>
        <issuerTradingSymbol>TEX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001193675</rptOwnerCik>
            <rptOwnerName>CHOLMONDELEY PAULA H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TEREX CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>200 NYALA FARM ROAD</rptOwnerStreet2>
            <rptOwnerCity>WESPORT</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06880</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, par value $.01</value>
            </securityTitle>
            <transactionDate>
                <value>2013-08-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2044</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>29.95</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>25955</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.01</value>
            </securityTitle>
            <transactionDate>
                <value>2013-08-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4019</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>29.90</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>29974</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Proceeds from the sale were used by the reporting person to purchase shares in the Issuer in a retirement plan for the benefit of the reporting person. The sale transaction and purchase transaction are matchable under Section 16(b) of the Securities Exchange Act of 1934. The reporting person agreed to pay to the Issuer the full amount of any profit realized in connection with such short-swing transactions, less transaction costs.  However, as the cost of shares purchased, including transaction costs, exceeded the proceeds from the shares sold, there were no short-swing profits.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Scott J. Posner, by Power of Attorney</signatureName>
        <signatureDate>2013-08-30</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-08-29
0000097216
TEREX CORP
TEX
0001193675
CHOLMONDELEY PAULA H
C/O TEREX CORPORATION
200 NYALA FARM ROAD
WESPORT
CT
06880
1
0
0
0
Common Stock, par value $.01
2013-08-29
4
S
0
2044
29.95
D
25955
D
Common Stock, par value $.01
2013-08-29
4
P
0
4019
29.90
A
29974
D
Proceeds from the sale were used by the reporting person to purchase shares in the Issuer in a retirement plan for the benefit of the reporting person. The sale transaction and purchase transaction are matchable under Section 16(b) of the Securities Exchange Act of 1934. The reporting person agreed to pay to the Issuer the full amount of any profit realized in connection with such short-swing transactions, less transaction costs.  However, as the cost of shares purchased, including transaction costs, exceeded the proceeds from the shares sold, there were no short-swing profits.
/s/ Scott J. Posner, by Power of Attorney
2013-08-30