As filed with the Securities and Exchange Commission on October 1, 2013.

Registration No. 333-189402

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

Post-Effective Amendment No. 1

to

Form S-4

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

ACTAVIS PLC

(Exact name of registrant as specified in its charter)

70 Sir John Rogerson’s Quay

Dublin 2, Ireland

(862) 261-7000

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

Paul M. Bisaro

President and Chief Executive Officer

Actavis, Inc.

Morris Corporate Center III

400 Interpace Parkway

Parsippany, New Jersey 07054

(862) 261-7000

(Name, address, including zip code, and telephone number, including area code, of agent for service)

With copies to:

Approximate date of commencement of proposed sale of the securities to the public: As soon as practicable after this Registration Statement becomes effective and upon completion of the merger and the acquisition described in the enclosed joint proxy statement/prospectus.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box.  ¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  x

Registration Number: 333-189402

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 the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

(Check one):

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

Exchange Act Rule 13e-4(i) (Cross-Border Issuer Tender Offer)  ¨

Exchange Act Rule 14d-1(d) (Cross-Border Third-Party Tender Offer)  ¨

EXPLANATORY NOTE

This post–effective Amendment No. 1 to Actavis plc’s (formerly known as Actavis Limited) Registration Statement on Form S-4 (Registration No. 333-189402) originally filed with the Securities and Exchange Commission on June 18, 2013 (as amended by Amendment No. 1, filed July 31, 2013) (as so amended, the “Registration Statement”), is being filed for the sole purpose of replacing Exhibit 5.1: Form of Opinion of Matheson, previously filed with the Registration Statement, with a final, executed version of Exhibit 5.1, filed herewith. The Registration Statement is hereby amended, as appropriate, to reflect the replacement of such exhibit. This Registration Statement became effective upon filing with the Commission on July 31, 2013 in accordance with Rule 462(d) under the Securities Act.

Item 21. Exhibits and Financial Statement Schedules.

(a) The exhibits listed below in the “Exhibit Index” are filed as part of, or are incorporated by reference in, this joint proxy/registration statement.

SIGNATURES

Pursuant to the requirements of the Securities Act, the registrant has duly caused this post–effective Amendment No. 1 to the registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Parsippany, State of New Jersey, on the 1st day of October, 2013.

Pursuant to the requirements of the Securities Act of 1933, this post–effective Amendment No. 1 to the registration statement has been signed by the following persons in the capacities indicated below on October 1, 2013.

Exhibit Index