<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>3</documentType>
<periodOfReport>2004-01-12</periodOfReport>
    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001097313</issuerCik>
        <issuerName>GRANT PRIDECO INC</issuerName>
        <issuerTradingSymbol>GRP</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001184451</rptOwnerCik>
            <rptOwnerName>FITZGERALD MATTHEW D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1330 POST OAK BLVD</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 2700</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77056</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>Senior Vice President &amp; CFO</officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable>
<nonDerivativeHolding>
<securityTitle>
<value>Shares of stock</value>
</securityTitle>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>60000</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeHolding>
</nonDerivativeTable>
<derivativeTable>
<derivativeHolding>
<securityTitle>
<value>Stock options</value>
</securityTitle>
<conversionOrExercisePrice>
<footnoteId id="F1"/>
</conversionOrExercisePrice>
<exerciseDate>
<footnoteId id="F1"/>
</exerciseDate>
<expirationDate>
<footnoteId id="F1"/>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Common Stock</value>
<footnoteId id="F1"/>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>125000</value>
</underlyingSecurityShares>
</underlyingSecurity>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</derivativeHolding>
</derivativeTable>
<footnotes>
<footnote id="F1">In accordance with Mr. Fitzgerald&#039;s employment, the company has agreed to issue to him options to purchase 125,000 shares of stock within 90 days following his initial employment date.  As of the date of the filing of this form, the date of such issuance had not yet been determined.</footnote>
</footnotes>
<remarks></remarks>
<ownerSignature>
<signatureName>Philip A. Choyce, Attorney-in-Fact</signatureName>
<signatureDate>2004-01-20</signatureDate>
</ownerSignature>
</ownershipDocument>

