Press Association Sport understands the south-coast club have the option to buy Bertrand outright next summer.
Bertrand has been with the Blues since 2006 but has found action hard to come by, spending last season on loan at Aston Villa.
Bertrand said: “For me personally, this is another chance to play and a chance to join a fantastic club that’s moving in the right direction.
“With the set-up the club’s got, the manager it has coming in and the ethos it has, I thought this was a perfect match.
“The manager’s name speaks for itself, so to play under somebody like that will be good for me. His ethos and the way he plays matches my game.
“I had a lot of options and a lot of interest but, having met the people and spoken to the manager and seen what the club is about, in my mind this was the perfect decision."
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."
- Sports & Recreation