Specimen Unit Certificate

Exhibit 4.1

NUMBER UNITS
U-

SEE REVERSE FOR CERTAIN
DEFINITIONS

CUSIP

FIRSTMARK ACQUISITION CORP. III

UNITS CONSISTING OF ONE CLASS A ORDINARY SHARE AND
ONE-FIFTH OF ONE REDEEMABLE WARRANT, EACH WHOLE WARRANT ENTITLING THE HOLDER TO PURCHASE ONE CLASS A ORDINARY SHARE

THIS CERTIFIES THAT                          is the owner of                Units.

Each Unit (“Unit”) consists of one (1) Class A ordinary share, par value $0.0001 per share (“Class A Share”), of FirstMark Acquisition Corp. III, a Cayman Islands exempted company (the “Company”), and one-fifth (1/5) of one redeemable warrant (each whole warrant exercisable for one Class A Share) (the “Warrant”). Each whole Warrant entitles the holder to purchase one (1) Class A Share (subject to adjustment) for $11.50 per share (subject to adjustment). Only whole Warrants are exercisable. Each Warrant will become exercisable thirty (30) days after the Company’s completion of an initial merger, share exchange, asset acquisition, share purchase, reorganization or other similar business combination with one or more businesses (each a “Business Combination”) and will expire unless exercised before 5:00 p.m., New York City Time, on the date that is five (5) years after the date on which the Company completes its initial Business Combination, or earlier upon redemption or liquidation. The Class A Shares and Warrants comprising the Units represented by this certificate are not transferable separately prior to                     , 2021, unless Credit Suisse Securities (USA) LLC elects to allow separate trading earlier, subject to the Company’s filing of a Current Report on Form 8-K with the Securities and Exchange Commission containing an audited balance sheet reflecting the Company’s receipt of the gross proceeds of its initial public offering and issuing a press release announcing when separate trading will begin. No fractional Warrants will be issued upon separation of the Units and only whole Warrants will trade. The terms of the Warrants are governed by a Warrant Agreement, dated as of                     , 2021, between the Company and Continental Stock Transfer & Trust Company, as Warrant Agent, and are subject to the terms and provisions contained therein, all of which terms and provisions the holder of this certificate consents to by acceptance hereof. Copies of the Warrant Agreement are on file at the office of the Warrant Agent at One State Street, 30th Floor, New York, New York 10004, and are available to any Warrant holder on written request and without cost.

Upon the consummation of the Business Combination, the Units represented by this certificate will automatically separate into the Class A Shares and Warrants comprising such Units.

This certificate is not valid unless countersigned by the Transfer Agent and Registrar of the Company.

This certificate shall be governed by and construed in accordance with the internal laws of the State of New York.

Witness the facsimile signature of its duly authorized officers.

FIRSTMARK ACQUISITION CORP. III

The Company will furnish without charge to each unitholder who so requests, a statement of the powers, designations, preferences and relative, participating, optional or other special rights of each class of stock or series thereof of the Company and the qualifications, limitations, or restrictions of such preferences and/or rights.

The following abbreviations, when used in the inscription on the face of this certificate, shall be construed as though they were written out in full according to applicable laws or regulations:

Additional abbreviations may also be used though not in the above list.

For value received, ______________ hereby sell(s), assign(s) and transfer(s) unto ________________

PLEASE INSERT SOCIAL SECURITY OR
OTHER
IDENTIFYING NUMBER OF ASSIGNEE

(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

_______________Units represented by the within Certificate, and do(es) hereby irrevocably constitute and appoint

________________________Attorney to transfer the said Units on the books of the within named Company with full power of substitution in the premises.

Dated _______________

Signature(s) Guaranteed:

THE SIGNATURE(S) MUST BE GUARANTEED BY AN
ELIGIBLE GUARANTOR INSTITUTION (BANKS,
STOCKBROKERS, SAVINGS AND LOAN
ASSOCIATIONS AND CREDIT UNIONS WITH
MEMBERSHIP IN AN APPROVED SIGNATURE
GUARANTEE MEDALLION PROGRAM, PURSUANT TO
S.E.C. RULE 17Ad-15 (OR ANY SUCCESSOR RULE)
UNDER THE SECURITIES EXCHANGE ACT OF 1934, AS
AMENDED).

In each case, as more fully described in the Company’s final prospectus dated                , 2021, the holder(s) of this certificate shall be entitled to receive a pro-rata portion of certain funds held in the trust account established in connection with the Company’s initial public offering only in the event that (i) the Company redeems the Class A Shares sold in its initial public offering and liquidates because it does not consummate an initial business combination by                , 2023, or by such later date approved by the Company’s shareholders in accordance with the Company’s amended and restated memorandum and articles of association, (ii) the Company redeems the Class A Shares sold in its initial public offering in connection with a shareholder vote to amend the Company’s amended and restated memorandum and articles of association (A) to modify the substance or timing of the Company’s obligation to allow redemption in connection with a Business Combination or redeem 100% of the Class A Shares sold in its initial public offering if it does not complete its initial business combination by                , 2021, or (B) with respect to any other provision relating to the holder(s) rights or pre-initial business combination activity, or (iii) if the holder(s) seek(s) to redeem for cash his, her, its or their respective Class A Shares in connection with a tender offer (or proxy solicitation, solely in the event the Company seeks shareholder approval of the proposed initial business combination) setting forth the details of a proposed initial business combination. In no other circumstances shall the holder(s) have any right or interest of any kind in or to the trust account.