Company: SION
Filing Date: 2025-02-07
Form Type: S-8
Source: 0001193125-25-022767
Chunk: 0

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: S-8
Chunk 0
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As filed with the Securities and Exchange Commission on February 7, 2025 Registration No. 333- UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM S-8 REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 Sionna Therapeutics, Inc. (Exact name of registrant as specified in its charter)

| Delaware                                  |     |         84-2801521 |
| (State or other jurisdiction of           
 incorporation or organization)            |     |   -I.R.S. Employer 
 Identification No. |
| 21 Hickory Drive, Suite 500               |     |              02451 |
| (Address of Principle Executive Officers) |     |          -Zip Code |

Sionna Therapeutics, Inc. 2020 Stock Option and Grant Plan Sionna Therapeutics, Inc. 2025 Stock Option and Incentive Plan Sionna Therapeutics, Inc. 2025 Employee Stock Purchase Plan (Full title of the plans) Michael Cloonan President and Chief Executive Officer Sionna Therapeutics, Inc. 21 Hickory Drive, Suite 500 Waltham, MA 02451 (Name and address for agent for service) (617) 819-2020 (Telephone number, including area code, of agent for service) Copies to: Gabriela Morales-Rivera William D. Collins Janet Hsueh 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-acceleratedfiler, 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-2of the Exchange Act.

| Large accelerated filer |     | ☐ |     | Accelerated filer         |     | ☐ |
| Non-accelerated filer   |     | ☒ |     | Smaller reporting company |     | ☒ |
|                         |     |   |     | Emerging growth company   |     | ☒ |

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