Garde has signed a three-year contract with the Major League Soccer outfit, who finished ninth in the Eastern Conference in 2017, failing to reach the play-offs.
That led to the departure of former boss Mauro Biello and the swoop for the former Lyon and Aston Villa coach, who has been handed significant power by the Canadian side.
"We are very happy and proud that Remi Garde is joining the Montreal Impact," club president Joey Saputo said.
"As player, assistant coach and head coach, he excelled by demonstrating his qualities as a leader and his knowledge of the game.
"His vision and his experience, also in the development of young players, will allow us to continue to improve our methods and work process at the academy. It really is a fresh start on many levels."
This is the first job for Garde, 51, since his reign at Villa ended in March 2016.