The 29-year-old is out of contract with the Hammers, and manager Sam Allardyce insists he could no longer guarantee the player regular football with Andy Carroll expected to join the club from Liverpool.
Allardyce said: "Carlton's contract has come to an end and we have decided not to renew that contract.
"Carlton's need at his age is to be playing football as often as he can. From our point of view we felt it better that he found pastures new.
"We can only thank him for his outstanding commitment and service to West Ham United over the last seven years."
Meanwhile, Allardyce revealed talks are continuing with midfielder Gary O'Neil over a new deal.
"For Gary, negotiations are ongoing and that will be done between the chairmen and his agent now," Allardyce added.