UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): January 6, 2016

PROTHENA CORPORATION PUBLIC

LIMITED COMPANY

(Exact name of registrant as specified in its charter)

Ireland

(State or other jurisdiction of incorporation)

Adelphi Plaza

Upper George’s Street, Dún Laoghaire

Co. Dublin, A96 T927, Ireland

011-353-1-236-2500

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On January 6, 2016, Prothena Corporation plc (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with RBC Capital Markets, LLC, UBS Securities LLC and Barclays Capital Inc., as representatives (the “Representatives”) of the several underwriters named therein (collectively, the “Underwriters”), pursuant to which the Underwriters agreed to subscribe for an aggregate of 2,250,000 ordinary shares of the Company, $0.01 par value per ordinary share (the “Offering”). The price to the public in this offering was $53.00 per ordinary share, and the Underwriters have agreed to subscribe for such ordinary shares from the Company at a price of $49.82 per ordinary share. Under the terms of the Underwriting Agreement, the Company has granted the Underwriters an option for 30 days to subscribe for up to an additional 337,500 ordinary shares.

The Offering was made under a prospectus supplement and related prospectus filed with the Securities and Exchange Commission pursuant to the Company’s effective shelf registration statement on Form S-3 (Registration No. 333-203258).

The Offering is expected to close on January 12, 2016, subject to customary closing conditions. The Company expects to receive net proceeds from the Offering of approximately $111.8 million, after deducting the Underwriters’ discount and estimated offering expenses payable by the Company, but excluding any exercise of the underwriters’ option to subscribe for additional ordinary shares.

Pursuant to the Underwriting Agreement, the Company agreed to indemnify the Underwriters against certain liabilities, including liabilities under the Securities Act of 1933, as amended, or to contribute to payments that the Underwriters may be required to make because of such liabilities. The Company and all of the Company’s directors and executive officers have also agreed not to sell or transfer any ordinary shares held by them for 90 days after January 6, 2016 without first obtaining the written consent of RBC Capital Markets, LLC and UBS Securities LLC on behalf of the Underwriters, subject to certain exceptions.

A copy of the Underwriting Agreement is attached as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing descriptions of the Underwriting Agreement do not purport to be complete and are qualified in their entirety by reference to such exhibit.

A copy of the opinion of A&L Goodbody relating to the validity of the ordinary shares issued in the Offering is filed herewith as Exhibit 5.1.

(d) Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.