Kadir, 29, joined Marseille from Valenciennes in January, but has failed to establish himself in the first team at the Stade Velodrome.
Marseille have added to their attack further in the transfer window, bringing in winger Dimitri Payet from Lille and striker Saber Khelifa from Evian.
Algeria international Kadir made just 16 appearances for Marseille last term - 10 of which came from the bench - and has yet to start for the Elie Baup's unbeaten side this campaign.
He is likely to command more first-team football at Rennes, who have already added to their forward line with the loan capture of Nelson Oliveira from Benfica.
Rennes have an option to purchase Kadir should his loan spell prove to be a success, and manager Philippe Montanier - who took over from Frederic Antonetti after parting company with Real Sociedad in May - will hope that he can produce the goods as he aims to help the Brittany outfit improve on last season's 13th-place finish.
A statement on Marseille's official website read: "Olympique de Marseille and Stade Rennais have reached an agreement in principle for the loan, with an option, of Foued Kadir."
Rennes, who sit fifth in the Ligue 1 table, will play their next game on Saturday when they host Lille.
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