The 20-year-old has signed a five-year contract at Saint James' Park.
Livramento is Newcastle's third summer signing after Sandro Tonali and Harvey Barnes.
Southampton have made a significant profit on a player they signed from Chelsea in the summer of 2021 for just £5m.
Livramento made only two appearances last season as Southampton were relegated after suffering an ACL injury in April 2022.
He will provide cover and competition for the experienced Kieran Trippier at right back for the Magpies.
Livramento told the club’s official website: “I’m really excited to have signed for Newcastle United and I’m looking forward to playing in front of the fans.
“I was here for the games over the weekend; even though they were friendlies, there were so many fans and they were still so loud so I can’t wait to see what it feels like to play in front of them in a Premier League or Champions League game.
“The way the manager plays football is really attractive and I feel like it suits me as a player. And with how well the team did last season, the trajectory that we’re on now, I’m just looking forward to being a part of that.”
Newcastle manager Eddie Howe added: “Tino is a very talented young player with an exciting future ahead of him so we are delighted to sign him.
“At 20, he already has lots of attributes that I admire but he also has the potential and drive to really grow with this team. I’m looking forward to working with him and to seeing him develop in a Newcastle shirt.”
