The 25-year-old forward, who was a restricted free agent, avoided a salary arbitration hearing scheduled for Wednesday.
Sundqvist had an NHL career-high 31 points (14 goals, 17 assists) in 74 regular-season games with the St. Louis Blues last season and nine points (four goals, five assists) in 25 Stanley Cup Playoff games, helping them win the Cup for the first time in their history.
Selected by the Pittsburgh Penguins in the third round (No. 81) of the 2012 NHL Draft, Sundqvist has 40 points (16 goals, 24 assists) in 144 regular-season games with the Penguins and Blues, and nine points in 27 NHL playoff games.