The Maple Leafs also sent forward Pontus Aberg and defenseman Jesper Lindgren to the Penguins in exchange for Pittsburgh's first-round pick in the 2020 draft (15th overall), forwards Evan Rodrigues and Filip Hallander and defenseman David Warsofsky.
Kapanen, 24, was Pittsburgh's first-round pick (22nd overall) in the 2014 draft and was traded to Toronto in 2015 in the deal that sent forward Phil Kessel to the Penguins.
"Kasperi is a good, young player that brings speed to our lineup and plays the way we want to play," Penguins general manager Jim Rutherford said in a release. "Having previously drafted him, we know him as a player and feel he can improve our top-6. Aberg and Lindgren are both young players that will further add to our organizational depth."
Kapanen is signed through the 2021-22 season and his contract carries an average annual value of $3.2 million.
He tallied 36 points (13 goals, 23 assists) in 69 games with the Maple Leafs in 2019-20. The Finnish forward set career highs across the board with 44 points (20 goals, 24 assists) in 78 games in 2018-19.
Kapanen has played in 202 games, all for Toronto, and has 90 points (41 goals, 49 assists) and 55 penalty minutes. He has seven points (four goals, three assists) in 25 postseason contests.
Aberg, 26, recorded one assist in five games for the Leafs in 2019-20. He has 44 points (17 goals, 27 assists) in 132 games since 2016 with the Nashville Predators, Edmonton Oilers, Anaheim Ducks, Minnesota Wild and Toronto.
Lindgren, 23, was Toronto's fourth-round pick in 2015. He posted one goal and eight assists in 31 games with the AHL's Toronto Marlies in 2019-20.
Rodrigues, 27, had one goal in seven games with Pittsburgh after being acquired in a February trade from Buffalo. The Toronto native registered 71 points (26 goals, 45 assists) in 192 games with the Sabres from 2016-20.
Hallander, 20, was Pittsburgh's second-round pick in the 2018 draft and has played professionally in Sweden the last two seasons.
Warsofsky, 30, collected 33 points (10 goals, 23 assists) in 51 games for the AHL's Wilkes-Barre/Scranton Penguins in 2019-20. He has 11 points (two goals, nine assists) in 55 NHL games with the Boston Bruins, New Jersey Devils, Colorado Avalanche and Penguins.
--Field Level Media