Saints have completed the £12m signing of Celtic midfielder Victor Wanyama.
The 22-year-old Kenyan international joins the club on a four-year contract.
The fee for Wanyama makes his move the biggest transfer in Scottish football history.
It is also similar to the amount Saints paid to Bologna last summer for Gastón Ramírez, which was a club record.
Wanyama will become the first Kenyan player to feature in the Premier League.
With his signing completed, he is now set to travel with the rest of the first-team squad on their pre-season tour to Spain and Austria.
The transfer, which turned into a protracted and public saga, has taken some time to conclude.
After agreeing a fee with Celtic earlier this summer, talks broke down when personal terms were being discussed.
However, the move was resurrected last week and Saints were able to reach a suitable deal with his representatives.
Wanyama told the club's website: "I'm very grateful to be here and I'm looking forward to playing in the Premier League.
"I am happy to be a part of Southampton's squad. It means a lot because because it has been my dream to play in the Premier League and now I am here."
Earlier in the day, he was quoted in the Kenyan media as saying: “I wish to thank everyone who has made this deal happen.
“Obviously, there are many clubs that have enquired about my services over the past season, but it’s Southampton who kept on trying to acquire my services, and that trust is very important.
“I am looking forward to going there and giving my best both for the team and for myself as a player.”
Wanyama moved to Celtic two years ago for £900,000 from Belgian side Beerschot AC, where he made his breakthrough as a professional.
Since joining the Glasgow giants he has made 91 appearances in two seasons, scoring 13 goals and winning two SPL titles, as well as the division’s young player of the year award.
Wanyama has also been capped 25 times for Kenya and is now captain of his country.
The president of the Football Kenya Federation, Sam Nyamweya, last night congratulated the player on his transfer.
“As a federation we are extremely thrilled with this move by Victor, who has continued to be Kenya’s football ambassador due to his exploits on the pitch,” he said in a statement that was somewhat premature at the time.
“It is through his sportsmanship and leadership qualities that led to the federation appointing him as the Stars captain.
“He has always honoured his call up to the Harambee Stars and every Kenyan should be proud of this move.
“Victor has been a role model to upcoming youngsters in the game and we encourage them to emulate him.”