The 21-year-old, who made his Premier League debut in the Saints' 3-1 win over Burnley in October, will remain at the club until 2021.
"It feels really good, as you'd expect," the left-back told the club's official website. "I'm delighted to sign a new deal with the club, having been here for so long already.
"It has been years and years of hard work to get to this point. I'm really happy to be out on the pitch playing games for the first team.
"The last couple of months have been amazing for me and this new contract really just rounds that off.