The 24-year-old spent last season on loan at Aston Villa, and will fill the void left by Luke Shaw, who has moved to Manchester United.
"He is a top player who will bring a lot of quality to the team," manager Ronald Koeman told the club website (www.saintsfc.co.uk).
"He has experience at the highest level and is an exciting, attacking player, which is what I like to see in my full-backs."
Bertrand, capped twice by England, joined Chelsea in 2006 but has struggled for game time and has spent loan spells at six clubs before this latest move.
(Reporting by Josh Reich; Editing by John O'Brien)
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