The 31-year-old has signed a two-year contract with the Foxes.
L'homme de 31 ans, who won 19 international caps between 2012 et 2017 and travelled to Euro 2016, has signed a two-year contract with Brendan Rodgers’ Foxes
“I’m really happy to be here; joining Leicester really complemented my inner ambitions and what I still want to achieve in my career,” he told the club’s website.
“Leicester have built something amazing over the years, especially since Brendan Rodgers has come in.
“It’s fantastic to be a part of this fantastic club and I’m going to give my all to try and do the fans proud.”
Bertrand began his career with Chelsea, where he won the FA Cup, Champions League and Europa League.
The left-back made 240 appearances during seven seasons with Southampton after leaving Stamford Bridge in 2014, initially on loan.
He was sent off when Saints suffered a record-equalling 9-0 Premier League loss at home to new club Leicester in October 2019.