The midfielder has penned a deal to 2029 as he continues his injury recovery.
Palace chairman Steve Parish said: It is brilliant news that Cheick, who was such an outstanding performer for us last season, has signed this new contract.
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We have no doubt that, after such a fantastic start to life in red and blue, Cheick will continue to progress as a player in the years to come, and I couldn't be happier to see him become the latest player to commit their future to Crystal Palace."
Doucour also told the club's website: I'm really happy to commit my future to Crystal Palace I've enjoyed my time here so much already.
We have a great group here, and I'm looking forward to getting back on the pitch as quickly as possible to play in front of the supporters of this fantastic football club."