The 31-year-old full-back will complete his transfer from Arsenal on a three year contract following the World Cup in Brazil, where he is currently on duty with the French squad.
Sagna is a hugely experienced defender, having represented Arsenal 213 times since joining them from Auxerre in 2007. During that time he has picked up 41 caps for the French national side.
France are in Group E of the World Cup, alongside Switzerland, Ecuador and Honduras. They begin their World Cup campaign against Honduras on Saturday.
Sagna will join up with his new team-mates for the first time after the end of the World Cup, when the squad take in their tour to America.
He will take up the vacant squad number 3, the same number he wore for the Gunners.