The 19-year-old left-back is the latest of a series of graduates to come through Southampton's academy and he has broken into the club's first team this season.
And Targett is thrilled to have committed his future to the club he has supported since childhood.
"I'm delighted to finally put pen to paper," Targett told the club's official website.
"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.
"I just need to keep working hard in training to keep pushing him."
