Benedetto, 29, who has made five international appearances since his debut in 2017, has reportedly cost the French club 16 million euros ($17.9 million).
He scored 39 goals in 63 matches in three years, twice winning the league for Argentine giants Boca after spells with Tijuana and Club America in Mexico and Gimnasa, Arsenal and Defensa in his home country.
Following Spanish defender Alvaro Gonzalez he is Andre Villas-Boas’ second summer signing since the Portugese coach took the reins from Rudi Garcia ahead of the new campaign.
The Velodrome side start the new term by hosting Reims on Saturday in their fifth season outside of the Champions League.
In order to comply with UEFA’s financial fair play regulations Marseille have been forced to sell winger Lucas Ocampos and Clinton N’Jie and they chose against renewing Mario Balotelli’s contract.