UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): December 5, 2013

Casella Waste Systems, Inc.

(Exact Name of Registrant as Specified in Charter)

Registrant’s telephone number, including area code: (802) 775-0325

Not applicable

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Item 1.01. Entry into a Material Definitive Agreement.

On December 5, 2013, Casella Waste Systems, Inc. (the “Company”) entered into a membership interest purchase agreement (the “Purchase Agreement”) pursuant to which the Company and Louisiana-Pacific Corporation (“LP” and, together with the Company, the “Sellers”) each sold their 50% membership interests in U.S. Greenfiber, LLC (“Greenfiber”) to GF Investors, LLC (“Purchaser”), a wholly-owned subsidiary of Tenex Capital Partners, L.P., for a total consideration of $18.0 million in cash. The Company expects aggregate net cash proceeds to be approximately $7.0 million ($3.5 million for each Seller), after the working capital true-up and the repayment of indebtedness and other transaction expenses.

Subject to the limitations set forth in the Purchase Agreement, each Seller, severally and not jointly, has agreed to indemnify Purchaser, Greenfiber and their respective officers, managers, directors, shareholders, members, affiliates, employees and agents from and against any losses incurred or suffered by such persons arising out of breaches by Greenfiber or the Sellers as specified in the Purchase Agreement.

The foregoing description of the terms of the Purchase Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Purchase Agreement, a copy of which is attached to this Current Report on Form 8-K as Exhibit 10.1 and is incorporated herein by reference. A copy of the Company’s December 5, 2013 press release announcing the sale of Greenfiber is attached to this Current Report on Form 8-K as Exhibit 99.1 and incorporated herein by reference.

Item 9.01. Financial Statements and Exhibits.

SIGNATURE

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 hereunto duly authorized.

Exhibit Index