The Saints have paid a €12m (£10.9m) fee for Salisu which could prove to be a bargain if his form carries over.
— Southampton FC (@SouthamptonFC) August 12, 2020
Salisu made 32 appearances for Valladolid after becoming a regular last season, helping improve their defence.
Speaking about the new signing, Southampton boss Ralph Hasenhüttl said Salisu will fit in with the way his team plays.
“This is an important signing for us,” said Hasenhüttl. “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.”
A good club to develop
Salisu pointed out Southampton’s history of developing player as a factor in his move.
“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.
“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.”