Diame has agreed a three-year contract and will join the Hammers when his contract with Wigan ends on 1st July.
Diame was strongly linked with a move a move to Liverpool before the club sacked Kenny Dalglish.
The 25-year-old told his new club’s official website: “I’m very happy and very excited to have joined the club and I’m looking forward to the start of the season.
“I know the fans here get behind the team all the time and I’m looking forward to showing everybody what I can do on the pitch.”
West Ham boss Sam Allardyce said: “His potential is really good and he’s ready to go and compete in the first team straight away, as he’s had three years in the Premier League already.
“He’s 25 and can only get better and I hope he can go on to prove that to me and all of the West Ham United fans. He will be a very exciting addition to the squad.”
