Tottenham Hotspur have confirmed the signing of Toby Alderweireld, via their official website.
The Belgian international has agreed a deal until 2020, leaving La Liga club Atletico Madrid.
The defender was the subject of a protracted dispute between Spurs, Atletico and Southampton - who had the defender at the club last season and believed they were entitled to exercise a first option agreement to sign him.
Alderweireld sent a message on Twitter after the move was confirmed, expressing his delight, and also thanking the Saints.
The defender becomes Tottenham's third summer signing, following Kevin Wimmer and Kieran Trippier.
The transfer business demonstrates the club's clear aim to improve the backline as they seek to improve on their fifth placed finish last season, Mauricio Pochettino's first as head coach.
Alderweireld is a Belgian international who is close friends with new Spurs colleague Jan Vertonghen, and will be expected to have few problems settling into life at his new club.