Perraud has joined Saints from Ligue 1 side Stade Brest for an undisclosed fee, signing a contract that sees him at St Mary’s for four years.
“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.”
23-year-old Perraud will slot into left-back for Southampton, who lost Ryan Bertrand on a free transfer after his contract expired at the end of last season.
Perraud started his career with French side Nice, before moving to Brest in 2019 where he made 61 appearances and scored three goals. He has also been capped for the France U21 team on eight occasions.
Theo Walcott was Southampton’s first signing ahead of the upcoming 2021-22 Premier League season, joining on a free after his deal with Everton expired at the end of June.