The Marseille midfielder has signed a three-year contract at Upton Park, arriving for an undisclosed fee as manager Sam Allardyce's eighth signing of the summer so far
The France international has agreed a three-year contract at Upton Park, becoming manager Sam Allardyce's eighth signing of the summer.
Diarra has won 44 caps for his country and played in three matches at Euro 2012, including Les Bleus' opening fixture against England.
"I am very happy to be here at West Ham - everything is signed now which is good and I am excited to be here at this club and to be playing for them in the Premier League," Diarra told the club's official website.
"I know a lot about West Ham as a club and when I knew they were interested in signing me, I was very happy and knew it would be a good choice for me.
"We are a Premier League club and I know of players who were here before like Paolo Di Canio and Scott Parker, but of the players now in the team, I know Guy [Demel], Mo [Diame] and of course, Modibo Maiga, who played in the French league last year.
"The club has just been promoted and I am looking forward to a good challenge with the team ahead. I see this as my chance in the Premier League to do well with West Ham.
"I hope that my experience can help and the fans will see me give my all in the matches."
The 31-year-old joined Bayern Munich from minnows Louhans-Cuiseaux in 2000, but failed to make a first team appearance for the German club.
He then moved on to Liverpool, but again could not break into the senior side and has spent his entire career since in France.
After joining Lens in 2005 following a loan spell, Diarra went on to appear for Lyon and Bordeaux before joining Marseille.
He made 18 appearances in Ligue 1 for the south coast side last season having won the division with Lyon and Bordeaux in previous years.