UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 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): August 30, 2013

Gran Tierra Energy Inc.

(Exact name of Registrant as specified in its charter)

300, 625 - 11th Avenue S.W.

Calgary, Alberta, Canada T2R 0E1

(Address of principal executive offices)

(403) 265-3221

(Registrant’s telephone number, including area code)

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):

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

The disclosure set forth under Item 2.03 is incorporated by reference here.

On August 30, 2013, Gran Tierra Energy Inc. (“Gran Tierra”) and Gran Tierra Energy International Holdings Ltd., a wholly-owned indirect subsidiary of Gran Tierra (“GTEIH”), entered into a $300,000,000 credit facility pursuant to a Credit Agreement, dated as of August 30, 2013, by and among Gran Tierra, GTEIH, Wells Fargo Bank, National Association, as administrative agent and global coordinator, and the lenders from time to time party thereto (the “Credit Agreement”).  Availability under the Credit Agreement is determined by a proven reserves-based borrowing base, and remains subject to the satisfaction of conditions precedent set forth in the Credit Agreement.  Loans under the Credit Agreement will mature on August 30, 2016.  The initial borrowing base is $150,000,000, and the borrowing base will be re-determined semi-annually based on reserve evaluation reports, subject to a maximum of $300,000,000.  The Credit Agreement includes a letter of credit sub-limit of up to $30,000,000.  Amounts drawn down under the Credit Agreement bear interest, at Gran Tierra’s option, at a eurodollar rate plus a margin ranging from 2.25% per annum to 3.25% per annum, or an alternate base rate plus a margin ranging from 1.25% per annum to 2.25% per annum, in each case based on the Borrowing Base Utilization Percentage.  Undrawn amounts under the Credit Agreement bear interest at 0.875% per annum, based on the average daily amount of unused commitments.  A letter of credit participation fee of 0.250% per annum will accrue on the average daily amount of letter of credit exposure.  Under the terms of the Facility, Gran Tierra is required to maintain compliance with specified financial and operating covenants.

In connection with entering into the Credit Agreement, and as a condition precedent to borrowing loans thereunder, Gran Tierra, GTEIH, and certain of Gran Tierra’s other direct and indirect subsidiaries have entered into certain ancillary agreements (the “Related Agreements”), including but not limited to:

The full text of the Credit Agreement is filed as Exhibit 10.1 hereto.

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 hereunto duly authorized.

Dated: September 6, 2013

Exhibit Index