Initial Registration Statement for Employee Benefit Plan (Form S-8)

As filed with the Securities and Exchange Commission on March 12, 2026

Registration No. 333-   

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-8

REGISTRATION STATEMENT

Under

The Securities Act of 1933

MBX Biosciences, Inc.

(Exact name of registrant as specified in its charter)

MBX Biosciences, Inc.

11711 N. Meridian Street, Suite 300

Carmel, Indiana 46032

(317) 659-0200

(Address, including zip code and telephone number, including area code, of Registrant's principal executive offices)

MBX Biosciences, Inc. 2024 Stock Option and Incentive Plan

MBX Biosciences, Inc. 2024 Employee Stock Purchase Plan

(Full title of the plans)

P. Kent Hawryluk

President and Chief Executive Officer

MBX Biosciences, Inc.

11711 N. Meridian Street, Suite 300

Carmel, Indiana 46032

(317) 659-0200

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Mitchell S. Bloom

Edwin M. O'Connor

Goodwin Procter LLP

100 Northern Avenue

Boston, Massachusetts 02210

(617) 570-1000

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of "large accelerated filer," "accelerated filer," "smaller reporting company," and "emerging growth company" in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ☐

STATEMENT OF INCORPORATION BY REFERENCE

This Registration Statement on Form S-8 is being filed for the purpose of registering (i) an additional 2,246,397 shares of common stock, par value $0.0001 per share ("Common Stock"), of MBX Biosciences, Inc. (the "Registrant") to be issued under the Registrant's 2024 Stock Option and Incentive Plan (the "2024 Plan") and (ii) an additional 449,279 shares of Common Stock of the Registrant to be issued under the Registrant's 2024 Employee Stock Purchase Plan (the "2024 ESPP"), for which Registration Statements on Form S-8 (File No. 333-282106 and File No. 333-285846) relating to the same employee benefit plans are effective.

These additional shares are of the same class as other securities relating to the 2024 Plan and 2024 ESPP for which the Registrant's Registration Statements on Form S-8 (File No. 333-282106 and File No. 333-285846) filed with the Securities and Exchange Commission (the "Commission") on September 13, 2024 and March 17, 2025, respectively, are effective.

Pursuant to General Instruction E to Form S-8, the contents of the Registration Statements on Form S-8 (File No. 333-282106 and File No. 333-285846) filed with the Commission on September 13, 2024 and March 17, 2025, respectively, are incorporated by reference except to the extent supplemented, amended or superseded by the information set forth herein. Only those items of Form S-8 containing new information not contained in the earlier registration statements is presented herein.

Part II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

SIGNATURES

Pursuant to the requirements of the Securities Act, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Carmel, Indiana, on this 12th day of March, 2026.

POWER OF ATTORNEY AND SIGNATURES

KNOW ALL BY THESE PRESENT, that each individual whose signature appears below hereby constitutes and appoints each of P. Kent Hawryluk and Steven L. Hoerter, as such person's true and lawful attorney-in-fact and agent with full power of substitution and resubstitution, for such person in such person's name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this Registration Statement, and to file the same, with all exhibits thereto, and all documents in connection therewith, with the Commission granting unto each said attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as such person might or could do in person, hereby ratifying and confirming all that any said attorney-in-fact and agent, or any substitute or substitutes of any of them, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act, this Registration Statement has been signed by the following person in the capacities and on the date indicated.