The 25-year-old has been handed a three-year deal after being released by relegated Wigan Athletic.
Speaking to the official site after being unveiled Diame revealed his excitement at the challenge of keeping the Hammers in the Premier League next season.
”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.”