The fee has still not been released but is expected to total £13.5 million in cash and players.
The 23-year-old has been described as 'the new Jaap Stam' by officials at the Stade Velodrome, and the club are delighted at his arrival.
L'OM are also reported to be interested in Malaga midfielder Joaquim Agostino.
The 25-year-old Portguese star has also interested French rivals Paris Saint Germain in the past but Marseille are said to be leading the race for his signature.
