The Penguins sent center Evan Rodrigues, defenseman David Warsofsky, forward Filip Hallander and their 2020 first-round draft pick (15th overall) to Toronto for Kapanen, the rights to KHL forward Pontus Aberg and defenseman Jesper Lindgren.
"Kasperi is a good, young player that brings speed to our lineup and plays the way we want to play. Having previously drafted him, we know him as a player and feel he can improve our top-six," Pittsburgh GM Jim Rutherford said.
The Penguins selected Kapanen with the 22nd overall pick in 2014, before trading him to the Leafs in the blockbuster Phil Kessel trade of July 2015. Rutherford also drafted Kapanen's father, Sami, in 1995 while general manager of the Hartford Whalers.
"Super excited about that obviously," Kapanen said. "It's where it all began for me, which is pretty cool."
In four years with the Maple Leafs, Kapanen had 41 goals and 49 assists in 202 games. He hit the 20-goal mark for the first time in 2018-19. But he struggled in the NHL restart, as the Maple Leafs lost a qualifying-round series to the Columbus Blue Jackets. He had just two assists in the five-game series.
"Puck wasn't going in for me," Kapanen said. "Overall, my game was iffy."
The trade gives Toronto some salary-cap flexibility. Rodrigues is a restricted free agent. Hallander is on his rookie deal through 2022, and Warsofsky is on a rookie deal through 2021. Kapanen makes $3.2 million against the salary cap through 2022.
The Penguins, who also lost in the qualifying round, sought change after a disheartening elimination to the Montreal Canadiens. With the core of Sidney Crosby, Evgeni Malkin and Kris Letang all at least 33, Kapanen's arrival gives the Penguins some fresh legs to work with as they try to keep the championship window open.
Kapanen, who called it an "obvious honor" to play with Crosby and Malkin, considers himself a two-way player who can help the Penguins at both ends of the ice.
"Obviously, I love scoring goals, who doesn't," Kapanen said. "I love playing with the puck and making plays, but at the same time I want to forecheck and be physical and be that guy to kind of get in your face a little bit and knock you off your game."
The Associated Press contributed to this report.