Balotelli scored eight goals in 15 Ligue 1 matches in an entertaining stint with the club. He left when his contract expired at the end of last season.
Benedetto has taken the number nine shirt worn by the Italian, who has been linked with a switch to Flamengo.
Benedetto scored 30 goals in 34 appearances across his first two Superliga campaigns with Boca but was on target just twice in 15 outings in 2018-19 after returning from knee and Achilles injuries.
He played a starring role in Boca's run to the 2018 Copa Libertadores final, scoring in the second leg as arch-rivals River Plate won the trophy.
Marseille have endured a tough pre-season and coach Villas-Boas acknowledged star players could yet be sold to the Premier League following a friendly loss to Napoli.