"This is an important first signing for us," said Southampton boss Ralph Hasenhuttl.
"Clearly, left-back is an area in which we were looking to strengthen and we have tracked Romain for a long time because he fits our profile for this position very well."
Perraud, who played eight times for France U21, began his career with Nice before moving to newly promoted Brest in 2019, where he made 61 appearances, scoring three goals.
Southampton were on the lookout for a left-back after Ryan Bertrand decided to leave the club on a free transfer at the end of his contract.