The Ghanian centre-back, 21, has signed a four-year deal after Saints triggered his £10.9m buyout clause.
"It is a very good feeling to be a Southampton player. I am very happy to be part of the club and I am looking forward to starting here," Salisu told the club's website.
"Southampton is a club with a very rich history in developing young players, so it is a very good club for me to develop my skills and to learn a lot as a young player.
"I know that the team expects a lot from me and I'm going to try to give all I can to help the team."
T̶h̶a̶t̶'̶s̶ ̶n̶o̶t̶ ̶S̶a̶l̶i̶s̶u̶
This IS Salisu!
Can we have our mentions column back now, please? pic.twitter.com/jC4YEknoyq
— Southampton FC (@SouthamptonFC) August 12, 2020