The 31-year-old moves to East London from Olympic Marseille for an undisclosed fee believed to be around £3 million.
The 6’3” defensive midfielder bring with his an immense amount of experience and his physical presence in the centre of the park is expected to help Kevin Nolan and Mark Noble this season.
Diarra has been capped 44 times for France and looked impressive during the 2012 UEFA European Championships.