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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-06-01</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001140113</rptOwnerCik>
            <rptOwnerName>SACHS DAVID A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TEREX CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>500 POST ROAD EAST, SUITE 320</rptOwnerStreet2>
            <rptOwnerCity>WESTPORT</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>2006-06-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>91.45</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>123598</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option Right to Buy</value>
                <footnoteId id="F3"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>91.50</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2006-06-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3287</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2006-06-01</value>
            </exerciseDate>
            <expirationDate>
                <value>2016-06-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock, par value $.01</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3287</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3287</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents average sale price.  Shares sold at various prices ranging from $91.21 to $91.65 as follows (shares at dollar price per share): 1,000 @ $91.21; 1,400 @ $91.60; and 100 @ $91.65.</footnote>
        <footnote id="F2">Represents shares beneficially owned as of June 1, 2006.</footnote>
        <footnote id="F3">Stock Options granted pursuant to one of the Company's long-term incentive plans representing payment of annual director retainer.  The stock options were calculated based on the Company's closing price on January 3, 2006.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ David A. Sachs</signatureName>
        <signatureDate>2006-06-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2006-06-01
0000097216
TEREX CORP
TEX
0001140113
SACHS DAVID A
TEREX CORPORATION
500 POST ROAD EAST, SUITE 320
WESTPORT
CT
06880
1
0
0
0
Common Stock, par value $.01
2006-06-01
4
S
0
2500
91.45
D
123598
D
Stock Option Right to Buy
91.50
2006-06-01
4
A
0
3287
0
A
2006-06-01
2016-06-01
Common Stock, par value $.01
3287
3287
D
Represents average sale price.  Shares sold at various prices ranging from $91.21 to $91.65 as follows (shares at dollar price per share): 1,000 @ $91.21; 1,400 @ $91.60; and 100 @ $91.65.
Represents shares beneficially owned as of June 1, 2006.
Stock Options granted pursuant to one of the Company's long-term incentive plans representing payment of annual director retainer.  The stock options were calculated based on the Company's closing price on January 3, 2006.
/s/ David A. Sachs
2006-06-05