Casey, 31, played in only three games before suffering a season-ending torn biceps during the Broncos’ loss to the Tampa Bay Buccaneers.
Casey had no guaranteed money left on his contract, which was set to run through the 2022 season. The move saves the Broncos $11.87 million against the salary cap in 2021.
The Broncos had acquired Casey from the Tennessee Titans prior to the season for a seventh-round pick in the 2020 draft.
The 10-year veteran finished with 14 tackles and three passes defensed.
Casey was a five-time Pro Bowler during his tenure with the Titans. He was named a second-team All-Pro in 2013 after he posted 10.5 sacks.
He has 51 sacks, 507 tackles and 8 forced fumbles in 142 career games.
Casey is the second defensive player released by the Broncos this offseason as cornerback A.J. Bouye was released on Feb. 10.
The Broncos now have just over $42 million worth of salary cap space as free agency approaches.
ESPN’s Jeff Legwold contributed to this report.