19-year-old Targett has committed himself to the club until 2019, having broken into Saints’ first team this season.
The Eastleigh-born defender made his debut in a Capital One Cup victory over Millwall in August, and went on to start in the subsequent three rounds of the competition.
Targett, who has been with Southampton since the age of eight, also racked up his first Premier League appearance this season, coming off the bench in September’s home victory over Queens Park Rangers.
“Coming up through the Academy and signing a new five-year deal to take me up until I’m 24 is only a positive.
“Being a Southampton supporter, it means a lot more to be staying at the club because it’s where I want to spend my career.
“I want to get into the first team and to keep progressing by playing week-in week-out. Ryan Bertrand is doing a great job this season and has made that left-back spot his own, so I just need to keep working hard in training to keep pushing him.”
Manager Ronald Koeman was pleased to see Targett commit his future to Southampton, adding: “We are happy that he has signed his new contract because he is a young, talented player.
“He has developed in a very good way. He has been playing the League Cup games, which has been a very good experience for him. Young players need a chance to play – he has had that and taken advantage of that.
“He is still only 19 so he has to develop, but he is getting the chance to do that which is a good signal to other young players in the club.”