Kevin Weekes of the NHL Network first reported details of the deal.
Hornqvist was a strong secondary contributor for the Penguins after being acquired in a June 2014 trade from the Nashville Predators.
The 33-year-old Swede recorded 264 points (132 goals and 132 assists) across 407 regular-season appearances in six seasons with the Pens. He added 38 points (22 goals and 16 assists) in 66 playoff contests with the team, which won back-to-back Stanley Cup titles in 2016 and 2017.
His list of accolades also includes a gold medal with Sweden at the 2018 IIHF World Championship.
Hornqvist has three seasons left on his five-year, $26.5 million contract. He could slot in on the Panthers' top line alongside Aleksander Barkov and Jonathan Huberdeau to open the 2020-21 campaign.
dom luszczyszyn @domluszczyszyn
Hornqvist is old and worn down, but still likely provides some value in a middle six with his goal-scoring and play-driving ability. Matheson is younger, but hasn't shown enough to be considered a top four guy and is being paid too much for too long for what he provides. https://t.co/ZoF7Q9FADa
Meanwhile, Matheson possesses a booming shot that's allowed him to tally 33 goals over the past four seasons for Florida. He's also added 58 assists over that period.
The 26-year-old Canadian hasn't enjoyed the same type of success in terms of defending and the transition game, as highlighted by the play-driving numbers above.
Those concerns combined with his lucrative eight-year, $39 million contract, which runs through the 2025-26 season, creates some risk for Pittsburgh. The Penguins do gain $425,000 in salary-cap space in the short term as part of the swap, though.
The Pens already featured a bit of a logjam among left-shot defensemen, including Brian Dumoulin, Marcus Pettersson, Jack Johnson and Juuso Riikola, heading into next season, so it's tough to project where Matheson fits in right away. More moves are likely on the horizon.