The forward, who will turn 26 on Saturday, was a restricted free agent and avoided a salary arbitration hearing scheduled for July 30.
Kapanen had 32 points (11 goals, 21 assists) in 79 games for the Penguins last season and three assists in seven Stanley Cup Playoff games.
Selected by Pittsburgh in the first round (No. 22) of the 2014 NHL Draft, Kapanen has 152 points (63 goals, 89 assists) in 321 regular-season games for the Penguins and Toronto Maple Leafs, and 13 points (five goals, eight assists) in 38 playoff games.
The Penguins signed defenseman Kris Letang to a four-year contract July 7 and center Evgeni Malkin to a four-year contract July 12.
Pittsburgh finished third in the Metropolitan Division last season (46-25-11) and lost to the New York Rangers in the Eastern Conference First Round. The Penguins have qualified for the playoffs in 16 straight seasons, the longest current streak in the NHL.