Bertrand originally joined Saints on a season-long loan deal at the start of the campaign but, following a number of impressive performances, the 25 year-old has now moved to St Mary’s until 2019.
It is the defender’s first permanent move away from Stamford Bridge, having enjoyed seven loan spells since making his breakthrough with the London club.
Bertrand took in spells at Bournemouth, Oldham Athletic, Norwich City, Reading and Nottingham Forest before being given his chance at Chelsea.
He made his first Premier League start in April 2011, before featuring 14 times during the 2011/12 campaign, including in the 2012 UEFA Champions League Final, which Chelsea won by beating Bayern Munich on penalties.
Bertrand featured 38 times for Chelsea during the 2012/13 campaign in all competitions, scoring his first goal for the club in the Community Shield against Manchester City.
Following José Mourinho’s return to Stamford Bridge, Bertrand’s first-team opportunities were limited and he ended last season on loan at Villa Park with Aston Villa, making ten Premier League appearances.
After Ronald Koeman’s arrival at St Mary’s he moved quickly to secure the services of Bertrand on loan and, following 27 appearances in all competitions, including two goals, the defender has joined on a permanent basis.