UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

Amendment No. 1

For the Fiscal Year Ended December 31, 2015

Commission File Number: 001-36257

RETROPHIN, INC.

(Exact Name of Registrant as specified in its Charter)

760-260-8600

(Registrant’s telephone number, including area code)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    ¨  Yes    x  No

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    ¨  Yes    x  No

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.    x  Yes    ¨  No

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Website, 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).    x  Yes    ¨  No

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  x

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” and “smaller reporting company” in Rule 12b-2 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    x  No

State the aggregate market value of the voting and non-voting common equity held by non-affiliates computed by reference to the price at which the common equity was last sold, or the average bid and asked price of such common equity, as of the last business day of the registrant’s most recently completed second fiscal quarter. $1,146,903,448.

The number of shares of outstanding common stock, par value $0.0001 per share, of the Registrant as of February 24, 2016 was 36,508,852.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive Proxy Statement, filed with the Securities and Exchange Commission within 120 days after the end of the registrant’s fiscal year covered by this Annual Report on Form 10-K, with respect to the 2016 Annual Meeting of Stockholders, are incorporated by reference into Part III of this Annual Report on Form 10-K.

Explanatory Note

Retrophin, Inc. (the “Company”) is filing this Amendment No. 1 (the “Amendment”) to its Annual Report on Form 10-K for the year ended December 31, 2015, as filed with the Securities and Exchange Commission on February 26, 2016 (the “Original Filing”), for the sole purpose of amending Item 15 of Part IV of the Original Filing to include the consent of BDO USA, LLP, the Company’s independent accounting firm, attached herewith as Exhibit 23.2, which was inadvertently omitted from the Original Filing.

In addition, this Amendment further amends Item 15 of Part IV of the Original Filing to include new certifications by our principal executive officer and principal financial officer pursuant to Sections 302 and 906 of the Sarbanes-Oxley Act of 2002 as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended.

Except for the amendments described above, we have not modified or updated disclosures presented in the Original Filing in this Amendment. Accordingly, this Amendment does not reflect events occurring after the filing of the Original Filing or modify or update those disclosures affected by subsequent events. Information not affected by this Amendment remains unchanged and reflects the disclosures made at the time the Original Filing was filed. Therefore, this Amendment should be read in conjunction with any documents incorporated by reference therein and our filings made with the Securities and Exchange Commission subsequent to the Original Filing.

PART IV

No financial statements are filed with this Amendment. These items were included as part of the Original Filing.

No financial statement schedules are filed with this Amendment. These items were not applicable or the required information was shown in the financial statements or notes thereto in the Original Filing.

The exhibits listed in the Original Filing are required by Item 601 of Regulation S-K. Each management contract or compensatory plan or arrangement required to be filed as an exhibit to this report has been identified. A list of the exhibits filed with this Amendment is provided below.

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.