No fee was disclosed for the 26-year-old, although West Ham said he was the club's record signing.
The winger's contract runs until 2017, with an option for a further year, and he could make his Hammers debut against Swansea on Saturday.
"I'm delighted to have signed," he told the West Ham website.
"It's a fantastic club and I'm pleased to be here. It's a great club and it's got great history and tradition. I know about Bobby Moore and the history and tradition that go with that."
"It'll be straight in and hit the ground running. There's no time to think about it, you've just got to get on with it, which is good. I hope to train with the team today and then travel up to Swansea."
"Certainly from an attacking sense he is one of the best final-third crossers in the Premier League and his stats were very good at Wolverhampton Wanderers.
"He delivers in the business end of the field and creates a lot of chances and is a player that can give opposition defences an awful lot of trouble. We are hoping he will produce that for us."
