Former Houston Dynamo head coach Wilmer Cabrera was named Garde's replacement.
The decision comes during the team's three-game winless streak, which has left the Impact clinging on to the final playoff spot in the Eastern Conference.
"Because I have so much respect for Remi as a person and as a professional, it was a very difficult decision to make and it was well thought, but our latest series of failures in the past couple of months and the way the team acts on the field led to that change," Impact president and CEO Kevin Gilmore said in a statement.
"We hope to bring back confidence to this group of players for the last stretch of the season, to get a playoff spot and to perform in the Canadian Championship final. I would like to sincerely thank Remi Garde for his commitment and his professionalism with our club since he joined in November 2017. I wish him only but the best in the future."
Garde failed to reach the playoffs in 2018, coming up short on the final day of the season against the New England Revolution.
The first month of the 2019 campaign ended with an embarrassing 7-1 defeat to Sporting Kansas City, but the team appeared to be righting the ship. However, the Impact have seen a massive dip in form, winning just two of their last nine matches.
Cabrera's arrival in Montreal comes just over a week after he was fired by the Dynamo.
The Colombian led Houston to the Western Conference semifinals during his first season in 2016. However, the 51-year-old's reign came to an end after the Dynamo only managed one win in nine matches across all competitions.