Bertrand has been with the Chelsea since 2006 but has found action hard to come by, spending last season on loan at Aston Villa.

"For me personally, this is another chance to play and a chance to join a fantastic club that's moving in the right direction," said Bertrand.

Southampton are believed to have the option to buy the 24-year-old English player outright next summer.

Manager Ronald Koeman added: "I'm very happy to be able to bring Ryan in. 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."

Southampton have seen a succession of players leave the Premier League club since they finished eighth last season.

Adam Lallana, Rickie Lambert and Dejan Lovren have all moved to Liverpool, while Calum Chambers has joined Arsenal.

Bertrand is expected to fill in for Luke Shaw at left-back, following his move to Manchester United.