The 28-year-old England midfielder has signed a four-year contract with the Hammers and will wear the number 10 shirt.
Ward-Prowse is a long-time target of West Ham manager David Moyes and he has finally got his man. The midfielder is a set-piece specialist and he will add creativity and quality to the Hammers’ midfield.
He is also a very good passer and dribbler, and he will be a key player for West Ham in their quest to qualify for the Champions League next season.
Ward-Prowse said: “I’m delighted to be here at West Ham United. It’s a club I’ve admired for a long time and I’m excited to be a part of this team.
“I’ve spoken to the manager and I’m really excited about his plans for the team. I think we can achieve great things together.”
Moyes said: “James is a top-class midfielder and I’m delighted to have him here at West Ham United. He’s a great passer, a great dribbler and he’s a set-piece specialist.
“He’s also a very good leader and he will be a great addition to our squad.”
Ward-Prowse is West Ham’s second signing of the summer, following the arrival of Nayef Aguerd from Rennes. The Hammers are also interested in signing Jesse Lingard and Armando Broja.
With Ward-Prowse now on board, West Ham are looking like a serious contender for European qualification next season.