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Sionna Therapeutics, Inc.

For the period ended Jun 12, 2025. Filed Apr 29, 2025.

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What shareholders are asked to vote on at a company's meeting. It is where to find what directors and top executives are paid, and who owns the most shares.

Jun 12, 2025 · Filed Apr 29, 2025

SION$3.280.0%Stored close · as of Nov 6, 2026
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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No. )

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

☐Preliminary Proxy Statement
☐Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))
☒Definitive Proxy Statement
☐Definitive Additional Materials
☐Soliciting Material Pursuant to §240.14a-12

(Name of Registrant as Specified in its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check all boxes that apply)

☒No fee required.
☐Fee paid previously with preliminary materials.
☐Fee computed on table in exhibit required by Item 25(b) per Securities Exchange Act of 1934 Rules 14a-6(i)(1) and 0-11.

NOTICE OF 2025 ANNUAL MEETING OF STOCKHOLDERS

To be held June 12, 2025

Notice is hereby given that the 2025 Annual Meeting of Stockholders of Sionna Therapeutics, Inc. (the “Annual Meeting”) will be held online on June 12, 2025 at 10:00 a.m. Eastern Time. The Annual Meeting will be held virtually, and you may attend the meeting via the Internet atwww.virtualshareholdermeeting.com/SION2025, where you will be able to vote electronically and submit questions. You will need the 16-digit control number included with the Notice of Internet Availability of Proxy Materials being mailed to you separately in order to attend the Annual Meeting.

Stockholders of record at the close of business on April 15, 2025, the record date for the Annual Meeting, are entitled to notice of, and to vote at, the Annual Meeting or any adjournment or postponement of the Annual Meeting. The purpose of the Annual Meeting is the following:

1.To elect three class I directors to our board of directors, to serve until the 2028 annual meeting of stockholders and until his or her successor has been duly elected and qualified, or until his or her earlier death, resignation or removal;
2.To ratify the appointment of Deloitte & Touche LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2025; and
3.To transact any other business properly brought before the Annual Meeting or any adjournment or postponement of the Annual Meeting.

You can find more information on each of the matters to be voted on at the Annual Meeting, including information regarding the nominees for election to our board of directors, in the accompanying proxy statement. The board of directors recommends a vote “FOR” the election of each of the three nominees for class I directors and “FOR” the ratification of the appointment of our independent registered public accounting firm for the fiscal year ending December 31, 2025.

This year, we are 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 (the “Notice”) instead of a paper copy of the accompanying proxy statement and our Annual Report on Form 10-K for the fiscal year ended December 31, 2024 (the “2024 Annual Report”). We are mailing the Notice on or about April 29, 2025, and it contains instructions on how to access both the 2024 Annual Report and accompanying proxy statement (the “Proxy Materials”) 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.

Please visitwww.virtualshareholdermeeting.com/SION2025 to attend the Annual Meeting virtually via the Internet. You will not be able to attend the Annual Meeting in person.

Whether or not you expect to attend the Annual Meeting online, we encourage you to read the accompanying proxy statement and vote your shares as promptly as possible to ensure your representation and the presence of a quorum at the Annual Meeting on the Internet as described in the instructions included in the Notice by voting online atwww.proxyvote.com. If you requested and received a paper copy of the Proxy Materials, you can vote by signing, dating and returning the enclosed proxy card, calling 1-800-690-6903 and following the recorded instructions, or online atwww.proxyvote.com. If you vote your shares on the Internet or by telephone, you will need to enter the 16-digit control number provided in the Notice.

Your vote is important regardless of the number of shares you own. If you attend the Annual Meeting online, you may vote your shares during the Annual Meeting virtually via the Internet even if you previously voted your proxy. Your proxy is revocable in accordance with the procedures set forth in the proxy statement.

If your shares are held in “street name,” that is, held for your account by a broker or other nominee, you will receive instructions from the holder of record that you must follow for your shares to be voted.

By order of the board of directors,

/s/ Michael Cloonan

Michael Cloonan

President and Chief Executive Officer

Waltham, Massachusetts

April 29, 2025

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