The England international has just played 28 games for Chelsea since joining them in 2006 and has been shipped away seven times on loan. He also played in the Champions League final against Bayern Munich.
Bertrand spent the last season on loan at Aston Villa and has started his career at Southampton brilliantly. Bertrand scoredhis debut goal for Southampton against Queens Park Rangers.
He said: “The situation became clear to me before I went on loan to Aston Villa (last season). I was getting those 15, 20, 25, 30-plus games a season, until I hit a stumbling block.
“That’s when the alarm bells rang. Before that, I had a chance to move from the club, but I was advised that I didn’t need to do that.
“It all unravelled in front of my eyes, and certain things didn’t make sense in terms of what certain people had said to me. All I ask for is honesty, and I didn’t get that.
“I wanted to be in a situation where the club is looking for the same things. In a way, me as a person and Southampton as a football club are quite similar. We’re both striving to do great things.”
Liverpool wanted to sign Ryan Bertrand last summer, but a bid was rejected by Chelsea as they didn’t want him to join their rivals.