Southampton signed Ghana defender Mohammed Salisu from Real Valladolid in a £10.9 million ($14 million) deal on Wednesday.
Salisu has agreed a four-year contract and is the Premier League club's second close-season signing following their swoop for Tottenham defender Kyle Walker-Peters.
Saints boss Ralph Hasenhuttl has used the fee from Denmark midfielder Pierre-Emile Hojbjerg's move to Tottenham to rebuild his defence.
Salisu, 21, moved to La Liga side Valladolid from Ghana in 2017 and the centre-back quickly established himself as a first-team regular.
"This is an important signing for us, Mohammed is a player who fits our profile well," Hasenhuttl said.
"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."
Salisu revealed Southampton's track record of developing young players convinced him to move to St Mary's.
"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," he said.
"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."