The 24-year-old, who spent last season on-loan at Aston Villa, has made a total of 57 appearances for Chelsea since making his debut for the club against Birmingham in 2011.
It is understood that Southampton have also included a clause in the loan deal allowing them to buy the former Gillingham trainee at the end of the season.
Southampton manager Ronald Koeman told the club’s official website: “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.
“I’ve been very impressed with how Matt Targett is doing in pre-season, but he is still young and developing, which I am sure he will continue to do over the coming seasons.
“Left-back is an important position in the team because it is quite specialised in terms of the attributes needed to play there, so it is nice to get this signing completed because it gives us some stability.”
Bertrand added: “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.
“I’m looking to get games and to play regularly in the way and the formation that I like, and I’m sure that will benefit my game. I still think the best is yet to come from me.”