The World Cup winner has spent most of his career at Marseille, making a record 613 appearances for the club since his arrival from Le Havre in 2007.
“Olympique de Marseille and Steve Mandanda have decided to go their separate ways by mutual agreement,” the club said in a statement.
“OM would like to sincerely thank Steve for everything he has brought to the club.”
The 37-year-old was under contract with Marseille until 2024 but spent much of last season as second choice to Pau Lopez.
According to a source close to the matter, Mandanda underwent a medical in Rennes on Wednesday before finalising his move.
He will compete with Senegal international Alfred Gomis for the starting role at Rennes.
Mandanda, the long-time back-up to Hugo Lloris, has won 34 caps for France and was part of the winning World Cup squad in Russia four years ago.
The Kinshasa-born keeper helped Marseille win the last of their nine Ligue 1 titles in 2010. He joined Crystal Palace on a free transfer in 2016 but returned to Marseille a year later.