Bailly will make the move to the Stade Velodrome permanent for €6m at the end of the season if Marseille qualify for the Champions League and he reaches a certain number of appearances.
The 28-year-old joined Manchester United from Villarreal in the summer of 2016 for £30m and went on to make 113 appearances for the club.
He helped the club win the Europa League and Carabao Cup in his first season but his subsequent years in Manchester have been hampered by fitness issues.
Bailly only made seven appearances across all competitions last season after Raphael Varane joined from Real Madrid.
The arrival of Lisandro Martinez from Ajax Amsterdam this summer has pushed Bailly further down the pecking order, with captain Harry Maguire and Victor Lindelof also providing competition for places.
Marseille are currently third on the French Ligue 1 table after winning two and drawing one of their first three games.
Bailly has won 46 caps for Ivory Coast and was a key member of their 2015 Africa Cup of Nations winning squad.
