The Nigerian international said he's delighted with the move and eager to join up with the likes of fellow arrivals Alex Song and Nikica Jelavic and towering centre-forward Andy Carroll at Upton Park.
Moses told club's official website: "I am very pleased to have joined West Ham. They are a great Club and the atmosphere here is amazing with the fans.
"I want to add my quality to what West Ham already have and hopefuly we can do even better than last season. I am very pleased to work under the new manager.
"Andy Carroll is a great striker and has a lot of experience in the Premier League. All the forwards have got a lot of experience and I want to help them score more goals."