The Blues obtained Halak, 25, from Montreal in a June 17 trade that sent forwards Lars Eller and Ian Schultz to the Canadiens.
Halak produced a 26-13-5 record with a 2.40 goals-against average and five shutouts during the regular season for Montreal and helped them reach the Eastern Conference finals. He ranked ninth in the NHL in GAA and fourth in save percentage (.940).
"Jaroslav will be a big help for our organization moving forward," Blues General Manager Doug Armstrong said. "He has made a great commitment to the Blues as he enters the prime of his career."
For his four-year NHL career, Halak has a 56-34-7 record with a 2.62 GAA and nine shutouts in 101 career games.
St. Louis went 40-32-10 last season but missed the playoffs.
"I've been looking forward to playing in St. Louis ever since the trade was announced," Halak said. "I believe in my abilities and hope to do some good things for the Blues."