Montreal will become the league's 19th team and the third from Canada, joining Toronto FC and the Vancouver Whitecaps, one of two news clubs starting up next season.
"We are proud to welcome Montreal, the second largest French-speaking city in the world and a multicultural metropolis whose residents are deeply passionate about the world's game, to Major League Soccer," MLS commissioner Don Garber said in a statement.
Owned by Canadian businessman Joey Saputo and his family, Montreal Impact currently play in a temporary second-tier competition that was borne out of a feud between rival leagues.
They have been linked with a move to the MLS for a number of years but did not enter the league in the last round of expansion when Vancouver and Portland were admitted for 2011.
"I am extremely proud of the acquisition of an MLS team," Saputo said.
"This is what Montreal deserved: a great league within a great city."
The team is expected to retain the Impact moniker and will play at Saputo Stadium, which will undergo a C$23 million renovation to raise the seating capacity to 20,000 from 13,000.
A select number of games will also be played at Montreal's Olympic Stadium.
"I believe the structure we already have in place, the support, the fans, the stadium, the following, will allow us to hit the ground running in 2012," Saputo told a news conference in Montreal.
"Just like we intend to compete on the field with the best clubs in MLS, we know that our fans will compete in the stands with the likes of Seattle or Toronto."
The MLS began with 10 teams in 1993 then increased to 12 in 1998 but two clubs dropped out in 2002 when attendance began to fall and teams were under financial pressure.
The league has been steadily increasing ever since and the MLS is expected to add a 20th team for 2012 with several U.S. cities already expressing interest.
