MBX Biosciences, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the Securities

Exchange Act of 1934 (Amendment No. )

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☒ Definitive Proxy Statement

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NOTICE OF 2026 ANNUAL MEETING OF STOCKHOLDERS

To be held online on June 4, 2026

Notice is hereby given that the 2026 Annual Meeting of Stockholders, or Annual Meeting, of MBX Biosciences, Inc., is on June 4, 2026 at 8:00 AM Eastern Time. This year’s Annual Meeting will be a virtual meeting, which will be conducted via live webcast. You may attend the meeting virtually via the Internet by visiting www.virtualshareholdermeeting.com/MBX2026, where you will be able to vote electronically and submit questions. You will need the control number included with the Notice of Internet Availability of Proxy Materials being mailed to you separately in order to attend the Annual Meeting. The purpose of the Annual Meeting is the following:

To elect two class II directors to our board of directors, each to serve until the 2029 annual meeting of stockholders and until their successor has been duly elected and qualified, or until their earlier death, resignation or removal;

To ratify the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2026; and

To transact any other business properly brought before the Annual Meeting or any adjournment or postponement of the Annual Meeting.

The proposal for the election of directors relates solely to the election of two class II directors nominated by the board of directors.

Only MBX Biosciences, Inc. stockholders of record at the close of business on April 10, 2026, will be entitled to vote at the Annual Meeting and any adjournment or postponement thereof.

MBX Biosciences, Inc. is following the Securities and Exchange Commission’s “Notice and Access” rule that allows companies to furnish their proxy materials by posting them on the Internet. As a result, we are mailing to our stockholders a Notice of Internet Availability of Proxy Materials, or the Notice, instead of a paper copy of the accompanying proxy statement and our Annual Report for the fiscal year ended December 31, 2025, or 2025 Annual Report. We plan to mail the Notice on or about April 22, 2026, and it contains instructions on how to access both the 2025 Annual Report and accompanying proxy statement over the Internet. This method provides our stockholders with expedited access to proxy materials and not only lowers the cost of printing and distribution but also reduces the environmental impact of the Annual Meeting. If you would like to receive a print version of the proxy materials, free of charge, please follow the instructions on the Notice.

To vote your shares prior the Annual Meeting, you must visit www.proxyvote.com, which contains all the materials related to the Annual Meeting, including voting instructions. To be admitted to the Annual Meeting and vote your shares at that time, you must visit www.virtualshareholdermeeting.com/MBX2026 and provide the control number as provided in the Notice, or proxy card, or voting instruction form. Please see the “General Information” section of the proxy statement that accompanies this notice for more details regarding the logistics of the virtual Annual Meeting, including the ability of stockholders to submit questions during the Annual Meeting, and technical details and support related to accessing the virtual platform.

Your vote is important. Whether or not you expect to attend the virtual meeting, it is important that your shares be represented. To ensure that your vote is recorded promptly, please vote as soon as possible, even if you plan to attend the meeting, by submitting your proxy via the Internet at the address listed on the proxy card or by signing, dating and returning the proxy card. Even if you have voted by proxy, you may still change your vote at the virtual meeting.

Please note, however, that if your shares are held through a broker, bank or other nominee and you wish to vote at the meeting, you must obtain a proxy issued in your name from that record holder.

By order of the Board of Directors,

P. Kent HawrylukPresident and Chief Executive Officer

Carmel, Indiana

April 22, 2026

Table of Contents

i

PROXY STATEMENT

FOR THE 2026 ANNUAL MEETING OF STOCKHOLDERS

TO BE HELD ONLINE ON June 4, 2026

This proxy statement contains information about the 2026 Annual Meeting of Stockholders, or the Annual Meeting, of MBX Biosciences, Inc., which will be held online on June 4, 2026 at 8:00 AM Eastern Time. You may attend the Annual Meeting virtually via the Internet at www.virtualshareholdermeeting.com/MBX2026 where you will be able to vote electronically and submit questions. The board of directors of MBX Biosciences, Inc. is using this proxy statement to solicit proxies for use at the Annual Meeting. In this proxy statement, the terms “MBX,” the “Company,” “we,” “us,” and “our” refer to MBX Biosciences, Inc. The mailing address of our principal executive offices is MBX Biosciences, Inc., 11711 N. Meridian Street, Suite 300, Carmel, Indiana 46032.

All properly submitted proxies will be voted in accordance with the instructions contained in those proxies. If no instructions are specified, the proxies will be voted in accordance with the recommendation of our board of directors with respect to each of the matters set forth in this proxy statement and the accompanying proxy card. You may revoke your proxy at any time before it is exercised at the meeting by giving our corporate secretary written notice to that effect.

We are mailing to our stockholders a Notice of Internet Availability of Proxy Materials, or the Notice, and our Annual Report for the fiscal year ended December 31, 2025, or 2025 Annual Report, on or about April 22, 2026.

We are an “emerging growth company” under applicable federal securities laws and therefore permitted to conform with certain reduced public company reporting requirements. As an emerging growth company, we provide in this proxy statement the scaled disclosure permitted under the Jumpstart Our Business Startups Act of 2012, or the JOBS Act, including the compensation disclosures required of a “smaller reporting company,” as that term is defined in Rule 12b-2 promulgated under the Securities Exchange Act of 1934, as amended, the Exchange Act. In addition, as an emerging growth company, we are not required to conduct votes seeking approval, on an advisory basis, of the compensation of our named executive officers or the frequency with which such votes must be conducted. We will remain an “emerging growth company” until the earliest of (i) the last day of the fiscal year following the fifth anniversary of our initial public offering in September 2024; (ii) the last day of the fiscal year in which our total annual gross revenue is equal to or more than $1.235 billion; (iii) the date on which we have issued more than $1 billion in nonconvertible debt during the previous three years; or (iv) the date on which we are deemed to be a large accelerated filer under the rules of the Securities and Exchange Commission, or the SEC. Even after we are no longer an “emerging growth company,” we may remain a “smaller reporting company.”

Important Notice Regarding the Availability of Proxy Materials for

the Annual Meeting of Stockholders to be Held on June 4, 2026:

This proxy statement and our 2025 Annual Report to Stockholders are

available for viewing, printing and downloading atwww.proxyvote.com.

A copy of our 2025 Annual Report, as filed with the SEC, except for exhibits, will be furnished without charge to any stockholder upon written request to MBX Biosciences, Inc., 11711 N. Meridian Street, Suite 300, Carmel, Indiana 46032, Attention: Corporate Secretary. This proxy statement and our 2025 Annual Report are also available on the SEC’s website at www.sec.gov.

PROXY STATEMENT

FOR THE 2026 ANNUAL MEETING OF STOCKHOLDERS

GENERAL INFORMATION

Why are you holding a virtual Annual Meeting?

This year’s Annual Meeting will be a “virtual meeting” of stockholders. We have implemented the virtual format in order to facilitate stockholder attendance at our Annual Meeting. We have designed our virtual format to enhance, rather than constrain, stockholder access, participation and communication. For example, the virtual format allows stockholders to communicate with us during the Annual Meeting so they can ask questions of our board of directors or management.

How can I virtually attend the Annual Meeting?

To attend and participate in the Annual Meeting, stockholders will need to access the live webcast of the meeting. To do so, stockholders of record will need to visit www.virtualshareholdermeeting.com/MBX2026 and enter the control number provided in the Notice on the day of the meeting, June 4, 2026, at 8:00 AM Eastern Time, and beneficial owners of shares held in street name will need to follow the instructions provided in the voting instructions form by the broker, bank or other nominee that holds their shares.

The live webcast of the Annual Meeting will begin promptly at 8:00 AM Eastern Time on June 4, 2026. We encourage stockholders to login to this website and access the webcast up to 15 minutes prior to the start time of the Annual Meeting. You should allow ample time in advance of the meeting. Additionally, questions regarding how to attend and participate via the Internet can be answered by following the instructions included at www.proxyvote.com.

If you wish to submit a question during the Annual Meeting, you will be able to do so during the meeting at www.virtualshareholdermeeting.com/MBX2026 and following the instructions there. Our Annual Meeting will be governed by the Annual Meeting’s Rules of Conduct and Procedures, which will address the ability of stockholders to ask questions during the meeting and rules for how questions will be recognized and addressed. The Annual Meeting’s Rules of Conduct and Procedures will be available on www.virtualshareholdermeeting.com/MBX2026 during the Annual Meeting.

How can I get help if I have trouble checking in or listening to the meeting online?

If you encounter any difficulties accessing the virtual meeting during the check-in or meeting time, a technical assistance phone number will be made available on www.virtualshareholdermeeting.com/MBX2026 up to 15 minutes prior to the start time of the meeting.

When are this proxy statement and the accompanying materials scheduled to be sent to stockholders?

We are mailing to our stockholders a Notice instead of a paper copy of the accompanying proxy statement and our 2025 Annual Report on or about April 22, 2026, and it contains instructions on how to access those documents over the Internet. You will need the control number included on the Notice, proxy card or voting instruction form to access these materials. If you would like to receive a print version of the Proxy Materials, free of charge, please follow the instructions on the Notice.

Who is soliciting my vote?

Our Board of Directors, or the board of directors, is soliciting your vote for the Annual Meeting, including at any adjournments or postponements of the meeting.

When is the record date for the Annual Meeting?

The record date for determination of stockholders entitled to vote at the Annual Meeting is the close of business on April 10, 2026.

How many votes can be cast by all stockholders?

There were 47,570,485 shares of our voting common stock, par value $0.0001 per share, outstanding on April 10, 2026, all of which are entitled to vote with respect to all matters to be acted upon at the Annual Meeting. Each stockholder of record is entitled to one vote for each share of our common stock held by such stockholder. As the stockholder of record, you have the right to grant your voting proxy directly to the individuals listed on the proxy card or vote on your own behalf at our virtual Annual Meeting. There are no cumulative voting rights. No shares of preferred stock were outstanding as of April 10, 2026.

Who is entitled to vote?

Registered Stockholders. If shares of our common stock are registered directly in your name with our transfer agent, you are considered the stockholder of record with respect to those shares. As the stockholder of record, you have the right to grant your voting proxy directly to the individuals listed on the proxy card or vote on your own behalf at our virtual Annual Meeting. Throughout this proxy statement, we refer to these registered stockholders as “stockholders of record.”

Street Name Stockholders. If shares of our common stock are held on your behalf in a brokerage account or by a bank or other nominee, you are considered to be the beneficial owner of shares that are held in “street name,” and the proxy materials were forwarded to you by your broker or nominee, who is considered the stockholder of record with respect to those shares. As the beneficial owner, you have the right to direct your broker, bank or other nominee as to how to vote your shares. Beneficial owners are also invited to attend our virtual Annual Meeting. However, since a beneficial owner is not the stockholder of record, you may not vote your shares of our common stock on your own behalf at the Annual Meeting unless you follow your broker’s procedures for obtaining a legal proxy. Note you should also be receiving a voting instruction form for you to use from your broker. Throughout this proxy statement, we refer to stockholders who hold their shares through a broker, bank or other nominee as “street name stockholders.”

How do I vote?

If you are a stockholder of record, there are four ways to vote:

•

By Internet. You may vote atwww.proxyvote.com, 24 hours a day, seven days a week, by 11:59 PM Eastern Time on Wednesday, June 3, 2026. You will need the control number included on your Notice or proxy card.

•

During the Annual Meeting. You may vote virtually by attending the meeting throughwww.virtualshareholdermeeting.com/MBX2026. To attend the Annual Meeting and vote your shares, you must login with the control number located on your Notice or proxy card.

•

By Telephone. You may vote using a touch-tone telephone by calling 1-800-690-6903, 24 hours a day, seven days a week, by 11:59 PM Eastern Time on Wednesday, June 3, 2026. You will need the control number included on your Notice or proxy card.

•

By Mail. You may vote by completing and mailing your proxy card.

Even if you plan to attend the virtual Annual Meeting, we recommend that you also vote by proxy so that your vote will be counted if you later decide not to attend the Annual Meeting.

If you are a street name stockholder, you will receive voting instructions from your broker, bank or other nominee. You must follow the voting instructions provided by your broker, bank or other nominee in order to instruct your broker, bank or other nominee on how to vote your shares. Street name stockholders should generally be able to vote by returning an instruction card, or by telephone or on the Internet. However, the availability of telephone and Internet voting will depend on the voting process of your broker, bank or other nominee. As discussed above, if you are a street name stockholder, you may not vote your shares on your own behalf at the Annual Meeting unless you obtain a legal proxy from your broker, bank or other nominee. If you hold your shares in multiple accounts, you should vote your shares as described in each set of proxy materials you receive.

By Proxy

If you will not be attending the Annual Meeting, you may vote by proxy. You can vote by proxy over the Internet by following the instructions provided in the enclosed proxy card. Proxies submitted by mail must be received before the start of the Annual Meeting.

If you complete and submit your proxy before the Annual Meeting, the persons named as proxies will vote the shares represented by your proxy in accordance with your instructions. If you submit a proxy without giving voting instructions, your shares will be voted in the manner recommended by the board of directors on all matters presented in this proxy statement, and as the persons named as proxies may determine in their discretion with respect to any other matters properly presented at the Annual Meeting. You may also authorize another person or persons to act for you as proxy in a writing, signed by you or your authorized representative, specifying the details of those proxies’ authority. The original writing must be given to each of the named proxies, although it may be sent to them by electronic transmission if, from that transmission, it can be determined that the transmission was authorized by you.

If any other matters are properly presented for consideration at the Annual Meeting, including, among other things, consideration of a motion to adjourn the Annual Meeting to another time or place (including, without limitation, for the purpose of soliciting additional proxies), the persons named in your proxy and acting thereunder will have discretion to vote on those matters in accordance with their best judgment. We do not currently anticipate that any other matters will be raised at the Annual Meeting.

How do I revoke my proxy?

You may revoke your proxy by (1) entering a new vote by mail that we receive before the start of the Annual Meeting or over the Internet or via telephone, (2) attending and voting at the Annual Meeting online (although attendance at the Annual Meeting will not in and of itself revoke a proxy), or (3) by filing an instrument in writing revoking the proxy or another duly executed proxy bearing a later date with our Corporate Secretary. Any written notice of revocation or subsequent proxy card must be received by our Corporate Secretary prior to the taking of the vote at the Annual Meeting. Such written notice of revocation or subsequent proxy card should be hand delivered to our Corporate Secretary or sent to our principal executive offices at MBX Biosciences, Inc., 11711 N. Meridian Street, Suite 300, Carmel, Indiana 46032, Attention: Corporate Secretary.

If a broker, bank, or other nominee holds your shares, you must contact such broker, bank, or nominee in order to find out how to change your vote.

How is a quorum reached?

Our Amended and Restated Bylaws, or bylaws, provide the holders of a majority in voting power of the outstanding shares of stock entitled to vote at, present in person or represented by proxy, will constitute a quorum for the transaction of business at the Annual Meeting.

Under the General Corporation Law of the State of Delaware, shares that are voted “abstain” or “withheld” and broker “non-votes” are counted as present for purposes of determining whether a quorum is present at the Annual Meeting. If a quorum is not present, the meeting may be adjourned until a quorum is obtained. There were 47,570,485 shares of common stock outstanding and entitled to vote on April 10, 2026, our record date. Therefore, a quorum will be present if 23,785,243 shares of our common stock are present in person or represented by executed proxies timely received by us at the Annual Meeting.

How is the vote counted?

Under our bylaws, any proposal other than an election of directors is decided by a majority of the votes properly cast for and against such proposal, except where a larger vote is required by law or by our Fourth Amended and Restated Certificate of Incorporation, or certificate of incorporation, or bylaws. Abstentions, votes withheld and broker “non-votes” are not included in the tabulation of the voting results on any such proposal and, therefore, do not have an impact on such proposals. A broker “non-vote” occurs when a nominee holding shares for a beneficial owner does not vote on a particular proposal because the nominee does not have discretionary voting power with respect to that item, and has not received instructions from the beneficial owner.

If your shares are held in “street name” by a brokerage firm, your brokerage firm is required to vote your shares according to your instructions. If you do not give instructions to your brokerage firm, the brokerage firm will still be able to vote your shares with respect to certain “discretionary” items, but will not be allowed to vote your shares with respect to “non-discretionary” items. Proposal No. 1 is a “non-discretionary” item. If you do not instruct your broker how to vote your shares with respect to this proposal, your broker may not vote for this proposal, and those shares will be counted as broker “non-votes.”

To elect our director and approve the other proposal being considered at the Annual Meeting, the voting requirements are as follows:

Shares voting “withheld” have no effect on the election of the directors (Proposal No. 1). Broker non-votes are not considered votes cast and will have no effect on the election of the directors.

Ratification of our independent registered public accounting firm (Proposal No. 2) is considered to be a discretionary item, and your brokerage firm will be able to vote your shares on this proposal even if it does not receive instructions from you. Abstentions and broker non-votes, if any, will not be counted “For” or “Against” Proposal No. 2 and will have no effect on Proposal No. 2.

Who pays the cost for soliciting proxies?

We are making this solicitation and will pay the entire cost of preparing and distributing our proxy materials and soliciting votes. If you choose to access the proxy materials or vote over the Internet, you are responsible for any Internet access charges that you may incur. Our officers and employees may, without compensation other than their regular compensation, solicit proxies through further mailings, personal conversations, facsimile transmissions, e-mails, or otherwise. Proxy solicitation expenses that we will pay include those for preparation, mailing, returning, and tabulating the proxies.

How may stockholders submit matters for consideration at an Annual Meeting?

The required notice must be in writing and received by our corporate secretary at our principal executive offices not less than 90 days nor more than 120 days prior to the first anniversary of the preceding year’s annual meeting. However, in the event that the date of the annual meeting is advanced by more than 30 days, or delayed by more than 60 days, from the first anniversary of the preceding year’s annual meeting, or if no annual meeting were held in the preceding year, a stockholder’s notice must be so received no earlier than the 120th day prior to such annual meeting and not later than the close of business on the later of (A) the 90th day prior to such annual meeting and (B) the tenth day following the day on which notice of the date of such annual meeting was mailed or public disclosure of the date of such annual meeting was made, whichever first occurs.

In addition, any stockholder proposal intended to be included in the proxy statement for the next annual meeting of our stockholders in 2027 must also satisfy the requirements of SEC Rule 14a-8 under the Securities Exchange Act of 1934, as amended, or the Exchange Act, and be received not later than December 23, 2026. In addition, to comply with the SEC’s new universal proxy rules, stockholders who intend to solicit proxies in support of director nominees other than our nominees must provide notice by the same deadline noted herein to present matters at the 2027 Annual Meeting of Stockholders. Such notice must comply with the additional requirements of Rule 14a-19(b). If the date of the annual meeting is moved by more than 30 days from the date contemplated at the time of the previous year’s proxy statement, then notice must be received within a reasonable time before we begin to print and send proxy materials. If that happens, we will publicly announce the deadline for submitting a proposal in a press release or in a document filed with the SEC.

How can I find out the results of the voting at the Annual Meeting?

We plan to announce preliminary voting results at the Annual Meeting. Final voting results will be published in a Current Report on Form 8-K, or Form 8-K, that we expect to file with the SEC within four business days after the Annual Meeting. If final voting results are not available to us in time to file a Form 8-K within four business days after the Annual Meeting, we intend to file a Form 8-K to publish preliminary results and, within four business days after the final results are known to us, file an additional Form 8-K to publish the final results.