Saints have signed Senegal international winger Sadio Mane in a deal worth £10m.
The 22-year-old has agreed a four-year contract with the club, joining from Austrian side Red Bull Salzburg.
Mane had emerged as Saints’ number-one wing target approaching deadline day, and the board have been able to wrap up the deal just in time.
Ronald Koeman told the club's website: “Right from the first meeting I had with Les Reed, when it was clear that we would have to change a lot, Sadio was on our list of potential players.
“I know the player because last season Ajax Amsterdam played against Red Bull Salzburg in Europe, and I saw him playing and I was impressed with his qualities and with the physicality that he showed.
“He can play in different positions in the attack – he can play on the left or right side and also as a number nine. It is incredible how many goals he has scored from his position, and I hope that he will do the same for us.
“With the way that we play and the fact that I know him very well, this is a good step forward for him.”
Mane began his career with French side Metz, coming through the academy system.
He made his first-team debut in the 2011/12 season and, after two impressive campaigns, he was signed by Austrian club Red Bull Salzburg.
Mane has made 88 appearances for the team, with an outstanding return of 46 goals.
He has been capped 15 times for the Senegal national side, while he also represented his country at the 2012 Olympic Games.