After weeks of trade rumors, on July 1, 2015, the Toronto Maple Leafs, who were entering a rebuilding phase, traded Kessel, Tyler Biggs, Tim Erixon, and a conditional second round draft pick to the Pittsburgh Penguins in a blockbuster deal for Kasperi Kapanen, Scott Harrington, Nick Spaling, and conditional first- (Sam Steel) and third-round (James Greenway) draft picks.
Toronto also retained $1.2 million of Kessel's salary for the remaining seven seasons of his contract.

Kessel scored his first regular season goal as a Penguin on October 10, while playing Arizona in the team's second game of the 2015–16 season.
On December 19, 2015, Kessel played his 500th consecutive game, becoming the 23rd player in league history to reach this milestone.

On June 12, 2016, Kessel won the Stanley Cup after defeating the San Jose Sharks in six games.
Kessel played on a line with Nick Bonino and Carl Hagelin that was the most effective line for the Penguins in the playoffs (called the HBK line).
Kessel led the Penguins in playoff scoring with 10 goals and 22 points and also led the team in shots with 98.
Kessel narrowly missed winning the Conn Smythe Trophy as playoff MVP, falling short of teammate and captain Sidney Crosby by three points.
In the 5-3-1 point distribution scheme based on the ballots of 18 voters, Crosby had nine first-place votes, five second-place votes, and three third-place votes, while Kessel had seven first-place votes, eight second-place votes, and one third-place vote.

As a member of the 2015–16 Stanley Cup winning team, Kessel was entitled to spend a day with the Stanley Cup.
 For his day with the cup, Kessel initially brought the cup to his hometown of Madison, Wisconsin, where he celebrated with family and friends.
 He then returned to Toronto, bringing the trophy to SickKids Hospital to share it with patients and their families.
When Kessel won his second championship trophy he took pictures of himself eating hot dogs out of the Stanley Cup.

In the 2017–18 season he scored 92 points, a career high, placed him eighth in the NHL.
He also posted a new career-best 58 assists in the season.

On October 11, 2018, during the third Pittsburgh game of the 2018–19 season, Kessel scored his sixth career hat-trick, which occurred as his first natural, and first as a Penguin.
From February 1 to March 5, 2019, Kessel did not score a goal in 16 games (however, he collected 11 assists), which was the longest goal-drought in his entire career.
On March 19, 2019, Kessel played in his 320th consecutive game for the Penguins, passing Craig Adams for the longest in their history.
After the last game of the regular season, Kessel was second in points for Penguins, and led the league with 10 game-winning goals.

Kessel and the Penguins were swept by the New York Islanders in the first round of the 2019 Stanley Cup Playoffs in what would be Kessel's final postseason with the Penguins; he scored a goal to go with an assist in the four-game loss.
After weeks of trade rumors, on July 1, 2015, the Toronto Maple Leafs, who were entering a rebuilding phase, traded Kessel, Tyler Biggs, Tim Erixon, and a conditional second round draft pick to the Pittsburgh Penguins in a blockbuster deal for Kasperi Kapanen, Scott Harrington, Nick Spaling, and conditional first- (Sam Steel) and third-round (James Greenway) draft picks. Toronto also retained $1.2 million of Kessel's salary for the remaining seven seasons of his contract.
Kessel scored his first regular season goal as a Penguin on October 10, while playing Arizona in the team's second game of the 2015–16 season. On December 19, 2015, Kessel played his 500th consecutive game, becoming the 23rd player in league history to reach this milestone.
On June 12, 2016, Kessel won the Stanley Cup after defeating the San Jose Sharks in six games. Kessel played on a line with Nick Bonino and Carl Hagelin that was the most effective line for the Penguins in the playoffs (called the HBK line). Kessel led the Penguins in playoff scoring with 10 goals and 22 points and also led the team in shots with 98. Kessel narrowly missed winning the Conn Smythe Trophy as playoff MVP, falling short of teammate and captain Sidney Crosby by three points. In the 5-3-1 point distribution scheme based on the ballots of 18 voters, Crosby had nine first-place votes, five second-place votes, and three third-place votes, while Kessel had seven first-place votes, eight second-place votes, and one third-place vote.
As a member of the 2015–16 Stanley Cup winning team, Kessel was entitled to spend a day with the Stanley Cup.  For his day with the cup, Kessel initially brought the cup to his hometown of Madison, Wisconsin, where he celebrated with family and friends.  He then returned to Toronto, bringing the trophy to SickKids Hospital to share it with patients and their families. When Kessel won his second championship trophy he took pictures of himself eating hot dogs out of the Stanley Cup.
In the 2017–18 season he scored 92 points, a career high, placed him eighth in the NHL. He also posted a new career-best 58 assists in the season.
On October 11, 2018, during the third Pittsburgh game of the 2018–19 season, Kessel scored his sixth career hat-trick, which occurred as his first natural, and first as a Penguin. From February 1 to March 5, 2019, Kessel did not score a goal in 16 games (however, he collected 11 assists), which was the longest goal-drought in his entire career. On March 19, 2019, Kessel played in his 320th consecutive game for the Penguins, passing Craig Adams for the longest in their history. After the last game of the regular season, Kessel was second in points for Penguins, and led the league with 10 game-winning goals.
Kessel and the Penguins were swept by the New York Islanders in the first round of the 2019 Stanley Cup Playoffs in what would be Kessel's final postseason with the Penguins; he scored a goal to go with an assist in the four-game loss.
Phil Kessel