Salisu moves to the Saint Mary’s stadium on a four-year deal after impressing in La Liga for Valladolid last season.
Salisu made 31 appearances and scored one goal in the Spanish top flight as Valladolid finished the 2019/2020 campaign in 13th place.
He joins right-back Kyle Walker-Peters as Ralph Hasenhuttl’s second summer signing ahead of the 2020/2021 campaign which begins on September 12.
"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."
"This is an important signing for us," Southampton manager Hasenhuttl added.
"Mohammed is a player who fits our profile well. He is young and has a great amount of potential, but he is also someone who has the qualities to come in and help the team as soon as he is up to speed with our way of playing.
"I like what I have seen of him so far in his games with Valladolid. He is strong defensively, he is calm with the ball - something that is important in our team - and he has good speed too.
"I think he will fit in well with us and that this will be a good place for him to develop his game further. We have some good centre-backs here and this will be another strong option for us."
Salisu is yet to be capped at international level for Ghana.
