UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 9)^*

STR HOLDINGS, INC.

(Name of Issuer)

Common Stock, par value $0.01 per share

(Title of Class of Securities)

78478V100

(CUSIP Number)

Red Mountain Capital Partners LLC

Attn: Willem Mesdag

10100 Santa Monica Boulevard, Suite 925

Los Angeles, California 90067

Telephone (310) 432-0200

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

December 20, 2013

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box.  ¨

Note. Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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This Amendment No. 9 amends and supplements the Schedule 13D filed with the Securities and Exchange Commission (the “SEC”) on August 15, 2011, as amended by Amendment No. 1 thereto, filed with the SEC on October 11, 2011, Amendment No. 2 thereto, filed with the SEC on February 1, 2012, Amendment No. 3 thereto, filed with the SEC on February 7, 2012, Amendment No. 4 thereto, filed with the SEC on February 24, 2012, Amendment No. 5 thereto, filed with the SEC on May 18, 2012, Amendment No. 6 thereto, filed with the SEC on August 21, 2012, Amendment No. 7 thereto, filed with the SEC on December 18, 2012, and Amendment No. 8 thereto, filed with the SEC on December 17, 2013 (together, this “Schedule 13D”), by (i) Red Mountain Capital Partners LLC, a Delaware limited liability company (“RMCP LLC”), (ii) Red Mountain Capital Partners II, L.P., a Delaware limited partnership (“RMCP II”), (iii) Red Mountain Partners, L.P., a Delaware limited partnership (“RMP”), (iv) RMCP GP LLC, a Delaware limited liability company (“RMCP GP”), (v) Red Mountain Capital Management, Inc., a Delaware corporation (“RMCM”), and (vi) Willem Mesdag, a natural person and citizen of the United States of America, with respect to the common stock, par value $0.01 per share (the “Common Stock”), of STR Holdings, Inc., a Delaware corporation (“STRI”). RMCP LLC, RMP, and RMCP GP are sometimes collectively referred to herein as “Red Mountain.” Red Mountain, RMCM and Mr. Mesdag are sometimes collectively referred to herein as the “Reporting Persons.” The filing of any amendment to this Schedule 13D (including the filing of this Amendment No. 9) shall not be construed to be an admission by the Reporting Persons that a material change has occurred in the facts set forth in this Schedule 13D or that such amendment is required under Rule 13d-2 of the Securities Exchange Act of 1934, as amended.

Items 5(a) and 5(b) of this Schedule 13D are hereby amended and restated as follows:

Because each of RMCP GP, RMCP LLC, RMCM and Mr. Mesdag may be deemed to control RMP, each of RMCP GP, RMCP LLC, RMCM and Mr. Mesdag may be deemed to beneficially own, and to have the power to vote or direct the vote, or dispose or direct the disposition, of all of the Common Stock beneficially owned by RMP.

Other than shares of Common Stock owned by RMP, none of the Reporting Persons or Mr. Teets or Mr. Watkinson may be deemed to own any shares of Common Stock.

Each of RMCP LLC, RMP and RMCP GP affirms membership in a group with each other but disclaims membership in a group with RMCM or Mr. Mesdag. Each of RMCM and Mr. Mesdag disclaims membership in a group with any person.

The filing of this Schedule 13D shall not be construed as an admission that any Reporting Person is the beneficial owner of any of the shares of Common Stock that such Reporting Person may be deemed to beneficially own. Without limiting the foregoing sentence, each of RMCM and Mr. Mesdag disclaims beneficial ownership of all shares of Common Stock reported in this Schedule 13D. In addition, the filing of this Schedule 13D shall not be construed as an admission that any partner, member, director, officer or affiliate of any Reporting Person is the beneficial owner of any of the shares of Common Stock that such partner, member, director, officer or affiliate may be deemed to beneficially own. Without limiting the foregoing sentence, each of Mr. Teets and Mr. Watkinson disclaims beneficial ownership of all shares of Common Stock reported in this Schedule 13D.

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Item 5(c) of this Schedule 13D is hereby amended to include the following information:

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SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: December 23, 2013

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