Exhibit 4.2


 


	
    NUMBER

     

    MCAE
	 	SHARES


 


MOUNTAIN CREST ACQUISITION CORP. III

INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE

COMMON STOCK


 


	 	 	
    SEE REVERSE FOR

CERTAIN 

DEFINITIONS

	 	 	 
	This Certifies 

that	 	CUSIP 62402U 107
	is the owner of	 	 


 


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

OF THE COMMON STOCK OF


 


MOUNTAIN CREST ACQUISITION CORP. III


 


transferable on the books of Mountain Crest
Acquisition Corp. III, a Delaware corporation (the “Corporation”) in person or by duly authorized attorney upon surrender
of this certificate properly endorsed. The Corporation will be forced to liquidate if it is unable to complete an initial business combination
within the period of time as set forth in the Corporation’s Amended and Restated Certificate of Incorporation, as the same may be
amended from time to time, all as more fully described in the Corporation’s final prospectus dated                     ,
2021


 


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


 


Witness the facsimile seal of the Corporation
and the facsimile signatures of its duly authorized officers.


 


	Dated:	 	 
	 	 	 
	CHAIRMAN	[INSERT SEAL HERE]	CHIEF EXECUTIVE OFFICER


 


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	 	 	 	 	 	 
	 	 	 	 	 	 	 	 	 
	JT TEN	—	as joint tenants with right of survivorship and not as tenants in common

         
	 	 	under Uniform Gifts to Minors Act
	 	 	 	 	(State)


  


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


  






     


     


    

 


Mountain Crest Acquisition Corp. III


 


The Corporation will furnish
without charge to each stockholder 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 Corporation 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 Corporation’s Amended and Restated Certificate of Incorporation and all amendments thereto and resolutions of the Board of
Directors providing for the issue of shares of Common Stock (copies of which may be obtained from the secretary of the Corporation), to
all of which the holder of this certificate by acceptance hereof assents.


 


For value received, ___________________________ hereby
sell, assign and transfer unto


 


 


	PLEASE INSERT SOCIAL SECURITY OR OTHER

IDENTIFYING NUMBER OF ASSIGNEE	 
	 	 
	 	 


 


 


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


shares of the capital stock represented by
the within Certificate, and do hereby irrevocably constitute and appoint 


 


____________________________________________________________________________ Attorney
to transfer the said stock on the books of the within named Corporation will full power of substitution in the premises.


 


	Dated	 	 	 	 
	 	 	 	 	 
	 	 	 	 	 
	 	 	 	Notice:	The signature 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:


 


	 	 
	THE SIGNATURE(S) SHOULD 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).	 


 


The holder(s) of this certificate shall be
entitled to receive a pro-rata portion of the funds from the trust account only in the event that (i) the Corporation is forced to
liquidate because it does not consummate an initial business combination within the period of time set forth in the
Corporation’s Amended and Restated Certificate of Incorporation, as the same may be amended from time to time (the
 “Charter”) or (ii) if the holder seeks to convert his shares upon consummation of, or sell his shares in a tender offer
in connection with, an initial business combination or in connection with certain amendments to the Charter. In no other
circumstances shall the holder(s) have any right or interest of any kind in or to the trust account.