UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q/A

(Amendment No. 1)

(Mark One)

For the quarterly period ended September 30, 2013

Or

For the transition period from                      to                     

Commission File No. 001-15795

RENTECH, INC.

(Exact name of registrant as specified in its charter)

10877 Wilshire Boulevard, Suite 600

Los Angeles, California 90024

(Address of principal executive offices)

(310) 571-9800

(Registrant’s telephone number, including area code)

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  x    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 (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).     Yes  x    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 the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

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

The number of shares of the Registrant’s common stock outstanding as of October 31, 2013 was 226,818,019.

EXPLANATORY NOTE

This Amendment No. 1 to the Quarterly Report on Form 10-Q for the quarterly period ended September 30, 2013 (the “Form 10-Q”), originally filed by Rentech, Inc. with the Securities and Exchange Commission on November 7, 2013, is being filed solely for the purpose of replacing Exhibit 10.1 with a new agreement that contains fewer redactions than the originally filed agreement.

Except as described above, no other changes have been made to the Form 10-Q. This Amendment No. 1 does not otherwise amend, change, modify or update the financial statements or disclosures set forth in the Form 10-Q as originally filed and does not otherwise reflect events, results or developments that may have occurred, or facts that have become known, after the original filing of the Form 10-Q. Accordingly, this Amendment No. 1 consists only of the cover page, this Explanatory Note, Item 6 of Part II to the Form 10-Q and the Signature Page.

Part II — OTHER INFORMATION

ITEM 6. EXHIBITS.

Exhibit Index

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 thereunto duly authorized.