Exhibit 4.2

 

NUMBER

 

NUMBER
C
SHARES
SEE
REVERSE FOR
CERTAIN
DEFINITIONS
CUSIP

 

FIRSTMARK ACQUISITION
CORP. III

 

INCORPORATED UNDER
THE LAWS OF THE CAYMAN ISLANDS
CLASS A ORDINARY SHARES

 

	This Certifies that

	 

	 


	 

	 

	 


	is the owner of

	 

	 




 

FULLY PAID AND NON-ASSESSABLE CLASS A
ORDINARY SHARES OF THE PAR VALUE OF $0.0001 EACH OF

 

FIRSTMARK ACQUISITION
CORP. III
(THE “COMPANY”)

 

transferable on the books of the Company
in person or by duly authorized attorney upon surrender of this certificate properly endorsed.

 

The Company will be forced to redeem all
of its Class A ordinary shares if it is unable to complete a business combination by                     ,
2023, or by such later date approved by the Company’s stockholders in accordance with the Company’s amended and restated
memorandum and articles of association, all as more fully described in the Company’s final prospectus dated                    ,
2021.

 

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

 

Witness the facsimile
signatures of its duly authorized officers.

 

	Secretary

 

	[Corporate Seal]
Delaware

 

	Principal Executive
Officer

 




 






     


     


    

 


 


FIRSTMARK ACQUISITION
CORP. III

 

The Company will furnish without charge
to each shareholder who so requests 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. This certificate and the shares represented thereby are issued and shall be held subject to all the provisions of the Company’s
amended and restated memorandum and articles of association and all amendments thereto and resolutions of the Board of Directors providing
for the issue of securities (copies of which may be obtained from the secretary of the Company), to all of which the holder of this certificate
by acceptance hereof assents. 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:



 




	TEN COM

	—

	as tenants in common

	 

	UNIF GIFT MIN ACT

	—

	 

	Custodian

	 


	 

	 

	 

	 

	 

	 

	(Cust)

	 

	(Minor)


	TEN ENT

	—

	as tenants by the entireties

	 

	 

	 

	 

	 

	 


	 

	 

	 

	 

	 

	 

	Under Uniform Gifts to Minors Act_____________

                                (State)


	JT TEN

	—

	as joint tenants with right of survivorship
and not as tenants in common

	 

	 

	 




 

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(S) OF ASSIGNEE(S))



 

 

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



 

 

 


 

 

 


 

 

Shares of the share capital represented
by the within Certificate, and do(es) hereby irrevocably constitutes and appoints



 

 

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



 

Dated:

 

 

 

NOTICE: THE SIGNATURE(S) TO THIS ASSIGNMENT
MUST CORRESPOND WITH THE NAME AS WRITTEN UPON THE FACE OF THE CERTIFICATE IN EVERY PARTICULAR, WITHOUT ALTERATION OR ENLARGEMENT OR ANY
CHANGE WHATEVER.

 

Signature(s) Guaranteed: 
By

 

 

 

 

 

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 public 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 public 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 ordinary 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 ordinary 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.