Exhibit 2.12







VIDEOTRON LTD. /
VIDÉOTRON LTÉE





THIRD SUPPLEMENTAL INDENTURE

Dated as of June 20, 2016





Wells Fargo
Bank, National Association,

Trustee















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THIRD SUPPLEMENTAL INDENTURE, dated as of June 20, 2016 (this Third
Supplemental Indenture), by and among Videotron Ltd. / Vidéotron Ltée, a corporation under the laws of the Province of Québec (the Corporation), 9176-6857 Québec inc., a corporation under the
laws of the Province of Québec (the Additional Subsidiary Guarantor) and Wells Fargo Bank, National Association, as trustee (the Trustee), to the Indenture, dated as of March 14, 2012, as
supplemented through the date hereof (the Indenture), by and among the Corporation, each of the subsidiary guarantors party thereto (collectively referred to as the Original Subsidiary Guarantors), and the
Trustee.

WHEREAS, the Corporation, the Original Subsidiary Guarantors and the Trustee have entered into the Indenture
governing the Corporations 5% Senior Notes due July 15, 2022 (the Notes);

WHEREAS,
Section 4.19 of the Indenture provides that under certain circumstances the Corporation shall cause a Restricted Subsidiary to execute and deliver to the Trustee a supplemental indenture providing for a Subsidiary Guarantee of the payment of
the Notes by such Restricted Subsidiary;

WHEREAS, the parties hereto are desirous of further supplementing the Indenture in
the manner hereinafter provided for the purpose of providing Subsidiary Guarantees by the Additional Subsidiary Guarantor in accordance with the terms of the Indenture;

WHEREAS, Section 9.01(e) of the Indenture provides that the Corporation and the Trustee may amend or supplement the Indenture without the consent of any Holder to add additional guarantees with respect to
the Notes;

WHEREAS, this Third Supplemental Indenture shall not result in a material modification of the Notes for purposes
of the Foreign Account Tax Compliance Act; and

WHEREAS, all things necessary have been done to make this Third Supplemental
Indenture a valid agreement of the Corporation, the Additional Subsidiary Guarantor and the Trustee, in accordance with its terms.

NOW,
THEREFORE, THIS THIRD SUPPLEMENTAL INDENTURE WITNESSETH:

For and in consideration of the premises contained herein, the
parties hereto mutually covenant and agree as follows:

1. Terms used in this Third Supplemental Indenture that are not
defined herein shall have the meanings set forth in the Indenture.

2. The Additional Subsidiary Guarantor hereby agrees to
provide an unconditional Subsidiary Guarantee on the terms and subject to the conditions and limitations set forth in the Indenture, including but not limited to Article 10 of the Indenture.

3. This Third Supplemental Indenture shall be construed as supplemental to the Indenture and shall form a part thereof, and the Indenture
is hereby incorporated by reference herein and, as supplemented, modified and restated hereby, is hereby ratified, approved and confirmed.



Third
Supplemental Indenture to VL 2012 Indenture



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4. This Third Supplemental Indenture shall be effective as of the date hereof. On and after
the date hereof, each reference in the Indenture to this Indenture, hereunder, hereof, or herein shall mean and be a reference to the Indenture as supplemented by this Third Supplemental Indenture
unless the context otherwise requires.

5. Except as provided below, in the event of a conflict between the terms and
conditions of the Indenture and the terms and conditions of this Third Supplemental Indenture, the terms and conditions of this Third Supplemental Indenture shall prevail.

6. If any provision of this Third Supplemental Indenture limits, qualifies or conflicts with another provision of the Indenture that is required to be included by the Trust Indenture Act of 1939, as
amended (the Act), as in force at the date this Third Supplemental Indenture is executed, the provision required by said Act shall control.

7. This Third Supplemental Indenture shall be governed and construed in accordance with the laws of the State of New York.

8. This Third Supplemental Indenture may be signed in any number of counterparts with the same effect as if the signatures to each counterpart were upon a single instrument, and all such counterparts
together shall be deemed an original of this Third Supplemental Indenture.

9. The recitals contained in this Third
Supplemental Indenture shall be taken as the statements of the Corporation, and the Trustee assumes no responsibility for their correctness. The Trustee makes no representations as to the validity or sufficiency of this Third Supplemental Indenture.

[SIGNATURES ON FOLLOWING PAGES]



Third
Supplemental Indenture to VL 2012 Indenture



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IN WITNESS WHEREOF, the parties hereto have caused this Third Supplemental Indenture to be
duly executed as of the day and year first above written.

CORPORATION:

VIDÉOTRON LTÉE



|       |  |                                                   |  |  |  |        |  |                             
|------ |  | ------------------------------------------------- |  |  |  | ------ |  | ----------------------------
|By:    |  | /s/ Hugues Simard                                 |  |  |  | By :   |  | /s/ Chloé Poirier           
|Name:  |  | Hugues Simard                                     |  |  |  | Name:  |  | Chloé Poirier               
|Title: |  | Senior Vice President and Chief Financial Officer |  |  |  | Title: |  | Vice President and Treasurer

ADDITIONAL SUBSIDIARY GUARANTORS:

9176-6857 QUÉBEC INC.



|       |  |                         |  |  |  |        |  |                             
|------ |  | ----------------------- |  |  |  | ------ |  | ----------------------------
|By:    |  | /s/ Hugues Simard       |  |  |  | By:    |  | /s/ Chloé Poirier           
|Name:  |  | Hugues Simard           |  |  |  | Name:  |  | Chloé Poirier               
|Title: |  | Vice President, Finance |  |  |  | Title: |  | Vice President and Treasurer

TRUSTEE:

WELLS FARGO BANK, NATIONAL ASSOCIATION



|       |  |                  
|------ |  | -----------------
|By:    |  | /s/ Yana Kislenko
|Name:  |  | Yana Kislenko    
|Title: |  | Vice President   



Third
Supplemental Indenture to VL 2012 Indenture