Southampton have signed Poland Under-21 defender Jan Bednarek from Lech Poznan on a five-year deal.
Meanwhile, England U21 central defender Jack Stephens, 23, has signed a new five-year contract.
Striker Sam Gallagher, 21, who got 11 Championship goals on loan at Blackburn last term, has signed a four-year deal.
Bednarek told the club website: "I am so happy that I could join a club like Southampton.
"This is the moment I have worked for that makes my dreams come true. I think it's a good step for me, because I have heard Southampton is such a good club for young players, so I can improve here."
Stephens began his career at Plymouth before moving to St Mary's and Southampton's vice-chairman of football Les Reed said: "His progression, and the level of his performances, was one of the real highlights of last season. We have been delighted with his development ever since he joined the club."
Regarding Gallagher he added: "The experience Sam gained last season at Blackburn was invaluable, and it was clear to see the further improvement that he made in his game during that time."
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