The details of the contract have not been released by Marseille, but the 25-year-old is reported to have signed a four-year deal at the Stade Velodrome.
Born in Marseille, the move sees Alessandrini return to his home town and he becomes the club's first major signing since Marcelo Bielsa was announced as their new coach in May.
Alessandrini joined Rennes in 2012 from Clermont Foot and consistently caught the eye with his strong performances at the Stade de Route de Lorient.
His form in in Brittany in 2012-13 earned him a call-up to the France squad for their February 2013 friendly with Germany, although he was an unused substitute for the game.
Alessandrini scored 20 goals in 60 appearances for Rennes, and will hope to take the next step in his career with Marseille.
Standard Liege striker Michy Batshuayi is expected to be confirmed as Marseille's next signing in the coming weeks.