Saints have completed the signing of Atletico Madrid defender Toby Alderweireld on a season-long loan.
The 25-year-old Belgian international, who can play at right-back and centre-half, underwent a medical this afternoon before agreeing terms with the club.
Saints are also understood to have an option to buy Alderweireld at the end of his loan spell for about £6m.
His signing is a coup for Saints and manager Ronald Koeman, who had identified him as their number-one defensive target in the closing stages of the window.
The club had plenty of competition for the player’s signature, with a host of Premier League clubs keen on him, but were able to persuade him to move to St Mary’s.
Alderweireld revealed he had been persuaded to join by Koeman.
“I’m very happy to be here," he told the club. “I had a good talk with the coach. First of all, it was very important for me to play this season and the coach gave me a good solution for that.
“The trainer likes how I play and really wanted me to come here, which is what convinced me to come. I really wanted a trainer that believes in me, because that’s important."
Koeman added: “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.
“We needed competition in our squad, and with Toby here we now have a lot of possibilities and quality.”
Alderweireld began his professional career at Ajax, making 185 appearances for the Dutch giants and winning three successive Eredevisie titles, from 2011 to 2013.
Atletico Madrid signed him last summer for a fee of about £6m, with the defender agreeing a four-year deal at the Spanish side.
He was part of the team that won the La Liga title last season, but made just 12 appearances in the competition, and 22 in total.
Alderweireld has been capped nearly 40 times for Belgium, and made four appearances in this summer’s World Cup, helping his country reach the quarter-finals.