Cahill joined Palace on a two-year deal in the summer of 2019 when his contract at the Chelsea expired.
The former England international made 47 appearances across all competitions for Palace during his time at Selhurst Park.
A statement published on Instagram from Cahill read: "After much thought and deliberation, I can confirm that I have left Crystal Palace FC after an enjoyable two years.
"I have really enjoyed my time at the club and I want to extend my sincere thanks to everyone involved at the club, especially the fans and staff who welcomed me into their South London family.
"It's been a privilege to play at Selhurst Park, especially in that first season where I got to experience the atmosphere that the Eagles fans create on a match day. Playing in front of the Holmesdale Road stand is a memory that I will cherish in particular.
"I would like to wish the club all the best for the future and wish them every success for the coming season."
Crystal Palace will be managed by former Arsenal captain Patrick Vieira next season.
The Eagles will face Chelsea in their opening game of the 2021/2022 season on August 14.
