The 26-year-old forward was a restricted free agent.
Lehkonen scored 13 points (seven goals, six assists) in 47 regular-season games with the Canadiens last season, and four points (three goals, one assist) in 17 Stanley Cup Playoff games.
Lehkonen scored the goal that put Montreal into the Stanley Cup Final, at 1:39 of overtime for a 3-2 win against the Vegas Golden Knights in Game 6 of the NHL Semifinals. The Canadiens lost the Cup Final to the Tampa Bay Lightning in five games.
Selected by Montreal in the second round (No. 55) of the 2013 NHL Draft, Lehkonen has scored 120 points (61 goals, 59 assists) in 338 regular-season games, and 12 points (six goals, six assists) in 33 playoff games.