In return, the Canadiens receive two youngsters: center Lars Eller and winger Ian Schultz. Eller, a center, was among the Blues' top prospects, while Schultz was a third-round pick in 2008.
"When I got the news, obviously I was surprised," Halak said from Slovakia on a conference call. "I'm very happy to be a part of a new organization, and I'm very excited about a new start. It's a new beginning."
"Jaroslav's play in this year's regular season and Stanley Cup playoffs was remarkable," Blues president John Davidson said in a statement. "He has had very impressive numbers in the NHL and we are thrilled that he will be wearing the Bluenote for us."
Halak will be a restricted free agent July 1 and has not signed a deal yet with the Blues, his agent, Allan Walsh, told ESPN.com.
"And I can add that Montreal never contacted us since the season ended," Walsh said.
Halak, 25, won a pair of Game 7s and helped Montreal bounce heavily favored Washington and Pittsburgh out of the playoffs. He won three straight games and eliminated the Capitals by stopping 131 of 134 shots.
But the Canadiens felt compelled to choose between Halak and Carey Price as their starting goaltender going forward and chose the younger Price.
"I was the one that got dealt," Halak said. "I'll try to make the new organization proud that they got me."
Halak, a Slovakian, was originally picked by Montreal in the ninth round (271st overall) of the 2003 entry draft. He was 26-13-5 last season and ranked fourth in the NHL in save percentage (.940) and ninth in goals against average (2.40), and tied for fifth with five shutouts.
St. Louis missed the playoffs this past season and was just 18-18-5 at home, among the worst marks in the NHL. Chris Mason got most of the starts in goal for the Blues, sharing time with Ty Conklin. The chance to acquire Halak persuaded the Blues to cease negotiations on a contract extension with the 34-year-old Mason, their starting goalie the past two seasons who had been due for unrestricted free agency.
Canadiens general manager Pierre Gauthier will receive two players who toiled mostly in the minors last season.
Eller, 21, played in seven games with the Blues in 2009-10, scoring twice. He spent most of time with the Blues' AHL affiliate in Peoria, Ill.
Schultz, 20, played for the Western Hockey League's Calgary Hitmen last season. His older brother, Jeff, plays with the Washington Capitals.
The Blues stopped well short of calling the deal a steal, forecasting a solid future for Eller.
"I think we've given up great value," Blues general manager Doug Armstrong said. "Lars was not a prospect we wanted to give up. It's a good deal for the Blues, and I think it's a good deal for Montreal."
The Blues also acquired forward T.J. Hensick from the Avalanche in exchange for forward Julian Talbot.
Pierre LeBrun covers the NHL for ESPN.com. Information from The Associated Press was used in this report.