Midfielder Mohamed Diame has turned down a new contract offer from Wigan to join West Ham on a three-year deal.
The 25-year-old Senegal international will sign for the Hammers on 1 July when his deal at Wigan expires.
Diame is West Ham's third signing since returning to the Premier League, following keepers Jussi Jaaskelainen and Stephen Henderson to Upton Park.
He said: "I am very excited to have joined the club. I am looking forward to showing what I can do on the pitch."
West Ham manager Sam Allardyce said: "His potential is really good and he is ready to go and compete in the first team straight away because he's had three years in the Premier League already."