The midfielder was among a number of players to be named on the Hammers' released list, ending a seven-season spell at the club for the 25-year-old.
The Wales international told the club's website: "As I move forward with my own career, I would like to take this opportunity to thank the fans once more. You have a fantastic set up, great staff and some wonderful players. As you move closer to the Olympic Stadium I wish you lots of luck.
"You deserve good times and I'm sure they are on the horizon. I'm sorry I didn't get chance to say goodbye but I hope I will in the near future.
"I have been lucky enough to work with some great people and during my time and have played alongside some truly inspirational and talented players."