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):April 23, 2013

Prothena Corporation plc

(Exact Name of Registrant as Specified in its Charter)

Registrant’s telephone number, including area code: (650) 837-8550

(Former Name or Former Address, if Changed Since Last Report.)

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 (see General Instruction A.2. below):

On April 23, 2013, Prothena Corporation plc (“Prothena”) issued a press release announcing the first dosing of patients in its Phase 1 clinical trial of its antibody therapeutic candidate, NEOD001. The full text of such press release is furnished as Exhibit 99.1 to this report.

The foregoing information in Item 7.01 of this Current Report on Form 8-K, together with the press release attached hereto as Exhibit 99.1, is being furnished pursuant to this Item 7.01 and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section, and it shall not be deemed incorporated by reference in any filing under the Securities Act or under the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such filing to Item 7.01 of this Current Report on Form 8-K.

By filing this Current Report on Form 8-K and furnishing this information, Prothena makes no admission as to the materiality of Item 7.01 in this report or the press release attached hereto as Exhibit 99.1. The information contained in the press release is summary information that is intended to be considered in the context of Prothena’s filings with the SEC and other public announcements that Prothena makes, by press release or otherwise, from time to time. Prothena undertakes no duty or obligation to publicly update or revise the information contained in this report, although it may do so from time to time as its management believes is appropriate. Any such updating may be made through the filing of other reports or documents with the SEC, through press releases or through other public disclosure.

On April 23, 2013, Prothena completed the first patient dosing in its Phase 1 clinical trial of its antibody therapeutic candidate, NEOD001. The study will evaluate the safety and tolerability of NEOD001 in patients with AL amyloidosis. NEOD001 is a monoclonal antibody targeting AL and AA amyloid for the potential treatment of amyloidosis.

(c) Exhibits.

SIGNATURES

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

EXHIBIT INDEX