The 24 year old had been linked with an £8m move to Liverpool, but has opted for a move to St Mary’s as he looks for regular first team football.
The South Coast club have lost five of their star performers from last season, with defenders Luke Shaw, Calum Chambers and Dejan Lovren, midfielder Adam Lallana and forward Rickie Lambert all moving on to pastures new.
Bertrand spent the second half of last season on loan at Aston Villa, and this is his seventh temporary move away from Stamford Bridge since turning professional.
The 24 year old has also had spells with Bournemouth, Oldham Athletic, Norwich City, Reading and Nottingham Forest.
He has made fifty-six appearances for Chelsea, one of which coming in the 2011/12 Champions League final victory over Bayern Munich.