Company: PRTA
Filing Date: 2025-08-08
Form Type: S-8
Source: 0001559053-25-000034
Chunk: 0

Company: PROTHENA CORP PUBLIC LTD CO
Filing Date: 2025-08-08
Form: S-8
Chunk 0
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As filed with the Securities and Exchange Commission on August 8, 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

Prothena Corporation Public Limited Company (Exact name of registrant as specified in its charter)

| Ireland                         |     | 98-1111119         |
| (State or other jurisdiction of 
 incorporation or organization)  |     | -IRS Employer      
 Identification No. |

77 Sir John Rogerson’s Quay, Block C Grand Canal Docklands Dublin 2, D02 VK60, Ireland (Address of Principal Executive Offices, including Zip Code) Prothena Corporation plc 2018 Long Term Incentive Plan, as amended (Full title of the plan) Michael J. Malecek Chief Legal Officer and Company Secretary Prothena Corporation plc c/o Prothena Biosciences Inc 1800 Sierra Point Parkway Brisbane, California 94005 (650) 837-8550 (Name, address, including zip code, and telephone number, including area code, of agent for service)

Copy to: Sharon Flanagan Sidley Austin LLP 555 California Street, Suite 2000 San Francisco, California 94104 (415) 772-1271

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, 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-2 of 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) of the Securities Act. ☐

PART I. INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

The information called for in Part I of Form S