The 25-year-old forward is coming off his best NHL season after recording 14 goals and 17 assists in 74 games in 2018-19. He also chipped in nine points in 25 playoff contests during the Blues' march to the Stanley Cup.
St. Louis acquired Sundqvist from the Pittsburgh Penguins in 2017 along with a first-round pick in exchange for Ryan Reaves and a second-rounder.
With Sundqvist locked up, the Blues now have roughly $4.3 million in available cap space, according to CapFriendly. Forward Ivan Barbashev and defenseman Joel Edmundson, both RFAs, are the club's only roster players currently without contracts.