Director of Football John Rudge said: "We've agreed to release Marcus from his contract. We weren't in the position to offer him the contract he wanted and so he's joined Coventry.
Tony Pulis added: "I want to thank Marcus for everything he has done for this football club.
"His contract was up at the end of this summer and so we couldn't stand in his way when Coventry came calling.
"As it happens we play Coventry this Saturday but there is a gentleman's agreement with Micky Adams that Marcus won't play."