UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q/A

(Mark One)

For the quarterly period ended March 31, 2018

Or

Commission file number: 001-35676

PROTHENA CORPORATION PUBLIC LIMITED COMPANY

(Exact name of registrant as specified in its charter)

Adelphi Plaza

Upper George’s Street

Dún Laoghaire

Co. Dublin, A96 T927, Ireland

(Address of principal executive offices including Zip Code)

Registrant’s telephone number, including area code: 011-353-1-236-2500

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  ☒    No  ☐

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  ☒    No  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company that prepares its financial statements in accordance with U.S. GAAP, 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 13(a) of the Exchange Act.  ☐

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ☐    No  ☒

The number of ordinary shares outstanding as of April 20, 2018 was 39,822,336.

EXPLANATORY NOTE

This Amendment No. 1 to the Form 10-Q (this “Amendment”) amends the Quarterly Report on Form 10-Q of Prothena Corporation plc for the period ended March 31, 2018 filed on May 9, 2018 (the “Form 10-Q”) for the sole purpose of refiling Exhibit 10.1(a), Exhibit 10.1(b), Exhibit 10.2 and Exhibit 10.3 (certain portions of which are omitted pursuant to a confidential treatment request filed with the Securities and Exchange Commission (the “SEC”)) and in connection therewith, to amend Part II, Item 6 of the Form 10-Q and the Exhibit Index to the Form 10-Q. In addition, as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended, new certifications by our principal executive officer and principal financial officer are filed as exhibits to this Amendment.

No other changes have been made to the Form 10-Q. This Amendment does not reflect events that may have occurred subsequent to the original filing date, and does not modify or update in any way the disclosures made in the original Form 10-Q.

EXHIBIT INDEX

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) 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, thereunto duly authorized.