The 25-year-old, who can play at right-back and centre-back, won La Liga with Atletico last season and played in their Champions League final defeat by Real Madrid.

He has won 37 international caps and was a member of the Belgium team that reached the quarter-finals of the World Cup in Brazil.

"I am delighted that we have been able to bring Toby to the club," said Southampton manager Ronald Koeman.

"He is a strong defender and is also very good at building up the play when we play out from the back.

"He played in the top league in Spain and against top teams in Europe, so he has a lot of confidence and experience, and he has a big future because he is still young."

Alderweireld began his professional career at Dutch giants Ajax and spent nine years at the Amsterdam club, winning three Eredivisie titles and one Dutch Cup.

He told the Southampton website: "I'm very happy to be here.

"I think this is an upcoming club. Southampton put a lot of effort in to get me, so now I want to give something back to the club by doing my best to help the team. I want to give everything for Southampton."

After finishing eighth in the Premier League last season, Southampton lost manager Mauricio Pochettino to Tottenham Hotspur and saw key players Adam Lallana, Luke Shaw, Rickie Lambert, Dejan Lovren and Calum Chambers leave.

But they have also brought in several new players, with Alderweireld following Dusan Tadic, Graziano Pelle, Fraser Forster, and Shane Long to St Mary's.