Sionna Therapeutics, Inc.

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:

(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)

NOTICE OF 2026 ANNUAL MEETING OF STOCKHOLDERS

To be held June 17, 2026

Notice is hereby given that the 2026 Annual Meeting of Stockholders of Sionna Therapeutics, Inc. (the “Annual Meeting”) will be held online on June 17, 2026 at 12:00 p.m. Eastern Time. The Annual Meeting will be held virtually, and you may attend the meeting via the Internet at www.virtualshareholdermeeting.com/SION2026, where you will be able to vote electronically and submit questions. In order to attend, you must register in advance atwww.proxyvote.com, prior to the deadline of June 16, 2026, at 11:59 p.m. Eastern Time. You will need the 16-digit control number included with the Notice of Internet Availability of Proxy Materials (the“Notice”) being mailed to you separately in order to attend the Annual Meeting.

Stockholders of record at the close of business on April 21, 2026, 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 four class II directors to our board of directors, to serve until the 2029 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 31,2026; 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 four nominees for class II directors and “FOR” the ratification of the appointment of our independent registered public accounting firm for the fiscal year ending December 31,2026.

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 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 31,2025 (the “2025 Annual Report”). We are mailing the Notice on or about April 27, 2026, and it contains instructions on how to access both the 2025 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.

The Annual Meeting will be conducted in virtual format only. To be admitted to the virtual Annual Meeting, you must register in advance atwww.proxyvote.com, using the 16-digit control number found on your proxy card or voting instruction form. Please note that the registration deadline is June 16, 2026 at 11:59 p.m. Eastern Time. You will not be able to attend the Annual Meeting unless you register by the deadline. Upon completing your registration, you will receive further instructions via e-mail that will allow you to access the Annual Meeting. 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 at www.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 register in advance and 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 27, 2026