Company: SION
Filing Date: 2025-04-29
Form Type: DEF 14A
Source: 0001193125-25-101830
Chunk: 0

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-04-29
Form: DEF 14A
Chunk 0
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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14AINFORMATION 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 Rule14a-6(e)(2)) |

| ☒ | Definitive Proxy Statement |

| ☐ | Definitive Additional Materials |

| ☐ | Soliciting Material Pursuant to§240.14a-12 |

Sionna Therapeutics, Inc. (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 Rules14a-6(i)(1)and0-11. |

Sionna Therapeutics, Inc. 21 Hickory Drive, Suite 500 Waltham, MA 02451 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 at www.virtualshareholdermeeting.com/SION2025, where you will be able to vote electronically and submit questions. You will need the 16-digitcontrol 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