Rennes are close to agreeing a deal worth £9m but are waiting for West Ham to sanction the transfer.
The 27-year-old enjoyed a prolific first season with the Hammers after he joined from Metz in 2014, but has since struggled to hold down a place in the starting XI due to injury problems.
Sakho has made 54 appearances in his three years at the club and scored an impressive 20 goals.
The Senegalese international was the subject of offers from Sunderland and West Bromwich Albion last summer but now looks set for a move back to France.