The 25-year-old moved to St Mary's on loan in the summer, but his impressive form - which has seen him return to contention for the senior England side - has prompted Ronald Koeman to tie the defender down.
Bertrand has signed a four-and-a-half-year deal upon leaving Chelsea, who he joined as a youngster 10 years ago, after making 22 appearances for the Saints this season.
The versatile defender has previously spent loan spells at the likes of Bournemouth, Norwich City and Nottingham Forest before joining Aston Villa for a short spell last season.
Bertrand made 56 appearances in total for Chelsea, most notably being handed his first start against Bayern Munich in the Champions League final in 2012 which the Blues won.
The Bertrand deal rounds off a productive transfer deadline day for Southampton, after the south coast club also landed Filip Djuricic on loan from Benfica earlier on Monday.
