Angel Di Maria is set to undergo a medical with Paris Saint-Germain in Doha on Monday ahead of a proposed £44m move from Manchester United.
Luton have signed former Peterborough and Brighton striker Craig Mackail-Smith on a two-year deal.
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 and 2011.
Rennes will be the sixth Ligue 1 side Diarra has played for, following Le Havre, Bastia, Lens, Lyon, Bordeaux and Marseille.