The Blues traded for the goalie earlier this offseason but still needed to lock him up.
Halak and Blues signed a 4-year $15 million deal with a $3.75 million yearly cap hit.
The Blues traded Lars Eller and Ian Schultz to the Canadiens for Halak who lead Montreal to upsets of Washington and Pittsburgh in the playoffs.
Halak was 26-13-5 last season, and had a .940 save percentage which was fourth in the NHL. Halak was also ninth in goals-against average with a 2.40, and also tied for fifth with five shutouts.