Diarra has signed a three-year contract in a deal believed to be in the region of £2 million.
"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," said Diarra.
"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 Hammers had earlier issued a statement on their official website in which they revealed they were on the brink of "tying up a major new signing in the next few hours".
A statement read: "West Ham United are hopeful of tying up a major new signing in the next few hours.
"The deal for the international star is at an advanced stage and it is hoped the transfer will be confirmed later on Friday."
Signing Diarra would be a coup for West Ham, with the midfielder one of France's better performers at Euro 2012, despite a disappointing season with Marseille.