The midfielder joined from Belgian club Anderlecht for £7m in summer 2014 and was part of the side which qualified for European football in both 2015 and 2016.
Speaking to Palace TV, he said: “I’m so happy. It’s been a long day but now everything is good and I’m very excited to start with my new teammates. This is the right time to move because I needed a new challenge. I have good memories of West Ham but I need to come here and give my best because I like the Crystal Palace project.
“I spoke to the manager and I know he’ll be a good manager. I also spoke to Pape Souare yesterday and he told me I had to come and now I’m here and I’m very happy.”