The Canadiens traded Halák on June 17, 2010 to the St. Louis Blues for Lars Eller and Ian Schultz. He played his first game with the Blues on October 9, 2010 where he earned a 2–1 overtime victory over the Philadelphia Flyers. Halák would collect his first shutout with the Blues on October 23, 2010 at home against the Pittsburgh Penguins, stopping 31 shots in a 1–0 overtime victory. In the 2011–2012 NHL season, Halak won the William M. Jennings Trophy with Brian Elliott for allowing the fewest amount of goals (165 collectively) in the regular season.