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

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2025-05-21</periodOfReport>

    <issuer>
        <issuerCik>0000097745</issuerCik>
        <issuerName>THERMO FISHER SCIENTIFIC INC.</issuerName>
        <issuerTradingSymbol>TMO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001216055</rptOwnerCik>
            <rptOwnerName>CASPER MARC N</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>168 THIRD AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WALTHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02451</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <officerTitle>Chairman &amp; CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>115173.623</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11300</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Alison Casper 2020 Irrevocable Trust</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14608</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Floral Park Associates, Inc.</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By MNC 2020 Irrevocable Trust</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Performance-based restricted stock unit</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2025-05-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>70769</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>70769</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>70769</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The reporting person disclaims beneficial ownership of the securities reported herein as indirectly beneficially owned, except to the extent of any pecuniary interest therein.</footnote>
        <footnote id="F2">The reporting person disclaims beneficial ownership of the securities reported herein as indirectly beneficially owned, except to the extent of any pecuniary interest therein.</footnote>
        <footnote id="F3">Represents a grant of performance-based restricted stock units, of which 70,769 will vest on May 21, 2030 subject to the terms and conditions of the grant.  The remainder of the award will vest, if at all, based upon the achievement of pre-established performance parameters relating to the Company's relative TSR performance against the TSR of the S&amp;P 500 Equal Weight Index, over a performance period beginning on May 1, 2025 and ending on December 31, 2029.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Melodie T. Morin, Attorney-in-Fact for Marc N. Casper</signatureName>
        <signatureDate>2025-05-22</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0508
4
2025-05-21
0000097745
THERMO FISHER SCIENTIFIC INC.
TMO
0001216055
CASPER MARC N
168 THIRD AVENUE
WALTHAM
MA
02451
1
1
Chairman & CEO
0
Common Stock
115173.623
D
Common Stock
11300
I
By Alison Casper 2020 Irrevocable Trust
Common Stock
14608
I
By Floral Park Associates, Inc.
Common Stock
5000
I
By MNC 2020 Irrevocable Trust
Performance-based restricted stock unit
2025-05-21
4
A
0
70769
0
A
Common Stock
70769
70769
D
The reporting person disclaims beneficial ownership of the securities reported herein as indirectly beneficially owned, except to the extent of any pecuniary interest therein.
The reporting person disclaims beneficial ownership of the securities reported herein as indirectly beneficially owned, except to the extent of any pecuniary interest therein.
Represents a grant of performance-based restricted stock units, of which 70,769 will vest on May 21, 2030 subject to the terms and conditions of the grant.  The remainder of the award will vest, if at all, based upon the achievement of pre-established performance parameters relating to the Company's relative TSR performance against the TSR of the S&P 500 Equal Weight Index, over a performance period beginning on May 1, 2025 and ending on December 31, 2029.
/s/ Melodie T. Morin, Attorney-in-Fact for Marc N. Casper
2025-05-22