Diarra, 31, joined West Ham last summer from Marseille, but picked up a thigh injury in training in September and has played just once for the club.
Diarra, who has 44 caps for France to his name, has spent most of his career in France, most notably in a successful spell at Bordeaux between 2007-11.
Rennes will be the sixth Ligue 1 side Diarra has played for, following Le Havre, Bastia, Lens, Lyon, Bordeaux and Marseille.
Diarra recently told French media: "I have a three-year contract. I'm not unhappy financially, but I came to play, not to take an early retirement.
"I am positively at breaking point. At all the clubs where I've been, I've always had a discussion with the coach, so I know what he expected of me, how the team plays, my positioning.
"But since my arrival at West Ham I never spoke with the manager. Every day is 'Hello, goodbye'. There is no dialogue, no explanation."
It now seems Allardyce has had a conversation with Diarra, which has resulted in the central midfielder leaving Upton Park.