Luke Shaw's big-money move to Manchester United meant Southampton were short of options on the left-hand side of defence but they have attempted to fill the void by bringing in the 24-year-old on loan for the 2014-15 campaign.
Bertrand had reportedly attracted interest from Liverpool but opted for a temporary switch to St Mary's to play under Ronald Koeman - his seventh loan spell away from Chelsea since joining the club in 2005.
"For me personally, this is another chance to play and a chance to join a fantastic club that's moving in the right direction," Bertrand told Southampton's official website.
"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."
Bertrand is Southampton's third signing of the close-season, alongside Dusan Tadic and Graziano Pelle.
New manager Koeman was delighted to get his man following the departures of full-backs Shaw and Calum Chambers, along with centre-back Dejan Lovren.
The club has also lost captain Adam Lallana and Rickie Lambert to Liverpool, as the side of 2013-14 has been broken up.
"He is a top player who will bring a lot of quality to the team," said Koeman. "He has experience at the highest level and is an exciting, attacking player, which is what I like to see in my full-backs.
"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."
