First team manager Ronald Koeman added: "I'm very happy to be able to bring Ryan in.
"I spoke to him on Monday afternoon and I was very impressed with his attitude and how keen he was to play games for Southampton.
"He is a top player who will bring a lot of quality to the team. He has experience at the highest level and is an exciting, attacking player, which is what I like to see in my full-backs."
The Bertrand loan move is welcome news for Southampton fans who have seen a succession of players leave the Barclays Premier League club since eighth place was secured in May.
Bertrand is expected to take Luke Shaw's place at left-back, following the latter's move to Manchester United. Adam Lallana, Rickie Lambert and Dejan Lovren have all moved to Liverpool, while Calum Chambers has joined Arsenal.
Southampton have pocketed almost £100 million ($169 million) for the combined deals, with chairman Ralph Krueger refuting reports that Jay Rodriguez and Morgan Schneiderlin would also be leaving.