Exhibit 2.12

VIDEOTRON LTD. / VIDÉOTRON LTÉE





SUPPLEMENTAL INDENTURE

Dated as of January 8, 2016





Computershare
Trust Company of Canada,

Trustee















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SUPPLEMENTAL INDENTURE, dated as of January 8, 2016 (this Supplemental
Indenture), by and among Videotron Ltd. / Vidéotron Ltée, a corporation under the laws of the Province of Québec (the Corporation), 9529454 Canada Inc., a corporation under the Canada Business
Corporations Act(9529454), 8480869 Canada Inc., a corporation under the Canada Business Corporations Act(8480869), Fibrenoire Inc., a corporation under the Canada Business Corporations Act(Fibrenoire) and Canadian P2P Fibre Systems Ltd. / Systèmes de Fibres P2P du Canada Ltée (Canadian P2P and, collectively with 9529454, 8480869 and Fibrenoire, the Additional Subsidiary
Guarantors, each an Additional Subsidiary Guarantor) and Computershare Trust Company of Canada (Computershare or the Trustee), as trustee, to each of (i) the Indenture, dated as of
July 5, 2011, as supplemented through the date hereof, by and among the Corporation, each of the subsidiary guarantors party thereto, and Computershare, as trustee (the 2011 Indenture), (ii) the Indenture, dated as of June 17,
2013, as supplemented through the date hereof, by and among the Corporation, each of the subsidiary guarantors party thereto, and Computershare, as trustee (the 2013 Indenture), and (iii) the Indenture, dated as of September 15,
2015, as supplemented through the date hereof, by and among the Corporation, each of the subsidiary guarantors party thereto, and Computershare, as trustee (the 2015 Indenture and collectively with the 2011 Indenture and the 2013
Indenture the Indentures and each an Indenture).

WHEREAS, the
Corporation, the existing subsidiary guarantors party thereto, and Computershare, as trustee, have entered into (i) the 2011 Indenture governing the Corporations 67/8% Senior Notes due 2021 (the 2021 Notes), (ii) the 2013 Indenture governing the Corporations
55/8% Senior Notes due June 15, 2025 (the 2025 Notes), and (iii) the 2015 Indenture governing the Corporations 53/4% Senior Notes due 2026 (the 2026 Notes and, collectively with the 2021 Notes and the 2025 Notes, the Notes);

WHEREAS, Section 4.19 of each of the Indentures, respectively, 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 issued thereunder by such Restricted Subsidiary;

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

WHEREAS, Section 9.01(5) of each Indenture, respectively, provides that the Corporation and the Trustee may amend or supplement such
Indenture without the consent of any Holder to add additional guarantees with respect to the notes issued thereunder; and

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

Supplemental Indenture  VL/Fibrenoire



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NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE WITNESSETH:

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

1. In respect of each the Indentures, respectively, terms used in this Supplemental Indenture that are not defined herein shall have the
meanings set forth in such Indenture.

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

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

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

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

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

7. This Supplemental Indenture shall be governed and construed in accordance with the laws of the Province of Quebec and the federal laws
of Canada applicable therein. The parties hereby acknowledge that they have expressly required this Supplemental Indenture be drawn up in the English language only. Les parties reconnaissent avoir expressément demandé que la
présente convention soit rédigée en anglais seulement.

8. This 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 Supplemental Indenture.

9. The recitals contained in this 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 Supplemental Indenture.

[SIGNATURES ON FOLLOWING PAGES]

Supplemental Indenture
 VL/Fibrenoire



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IN WITNESS WHEREOF, the parties hereto have caused this 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: |  |                                                          |  |  |  |    
|                                  |  |                                                          | 
|9529454 CANADA INC.               |  |                                                          |  |  |  |    
|                                  |  |                                                          |  | 
|By:                               |  | /s/ Hugues Simard                                        |  |  |  | By: |  | /s/ Chloé Poirier                   
|                                  |  | Name: Hugues Simard                                      |  |  |  |     |  | Name: Chloé Poirier                 
|                                  |  | Title: Vice President Finance                            |  |  |  |     |  | Title:: Vice President and Treasurer
|                                  |  |                                                          | 
|8480869 CANADA INC.               |  |                                                          |  |  |  |    
|                                  |  |                                                          |  | 
|By:                               |  | /s/ Hugues Simard                                        |  |  |  | By: |  | /s/ Chloé Poirier                   
|                                  |  | Name: Hugues Simard                                      |  |  |  |     |  | Name: Chloé Poirier                 
|                                  |  | Title: Vice President                                    |  |  |  |     |  | Title:: Vice President and Treasurer
|                                  |  |                                                          | 
|FIBRENOIRE INC.                   |  |                                                          |  |  |  |    
|                                  |  |                                                          |  | 
|By:                               |  | /s/ Hugues Simard                                        |  |  |  | By: |  | /s/ Chloé Poirier                   
|                                  |  | Name: Hugues Simard                                      |  |  |  |     |  | Name: Chloé Poirier                 
|                                  |  | Title: Vice President                                    |  |  |  |     |  | Title:: Vice President and Treasurer
|                                  |  |                                                          | 
|CANADIAN P2P FIBRE SYSTEMS LTD.   |  |                                                          |  |  |  |    
|                                  |  |                                                          |  | 
|By:                               |  | /s/ Hugues Simard                                        |  |  |  | By: |  | /s/ Chloé Poirier                   
|                                  |  | Name: Hugues Simard                                      |  |  |  |     |  | Name: Chloé Poirier                 
|                                  |  | Title: Vice President                                    |  |  |  |     |  | Title:: Vice President and Treasurer

Supplemental Indenture  VL/Fibrenoire



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|                                      |  |                                                        |  |  |  |     |  |                                                                
|------------------------------------- |  | ------------------------------------------------------ |  |  |  | --- |  | ---------------------------------------------------------------
|TRUSTEE:                              |  |                                                        |  |  |  |    
|                                      |  |                                                        | 
|COMPUTERSHARE TRUST COMPANY OF CANADA |  |                                                        |  |  |  |    
|                                      |  |                                                        |  | 
|By:                                   |  | /s/ Fabienne Pinatel                                   |  |  |  | By: |  | /s/ Ekaterini Galouzis                                         
|                                      |  | Name: Fabienne Pinatel                                 |  |  |  |     |  | Name: Ekaterini Galouzis                                       
|                                      |  | Title: Gestionnaire fiduciaire
Corporate Trust Officer |  |  |  |     |  | Title: Gestionnaire Fiduciaire Adjointe Associate Trust Officer

Supplemental Indenture  VL/Fibrenoire