The talented midfielder has left Montpellier in a deal worth an initial €9 million which could eventually rise to €12m with add-ons
Marseille completed a move for Montpellier midfielder Morgan Sanson on Tuesday.
Sanson, 22, was in his fourth season at Montpellier but the Frenchman has made the switch after the clubs agreed a deal of €9million, plus €3m in bonuses.
Marseille president Jacques-Henri Eyraud was thrilled to welcome Sanson, a France youth international.
"I am very pleased that the first player who joined us during this transfer window is one of the most promising talents in French football," he said.