The Belgium international, 26, has signed a five-year contract at White Hart Lane, becoming Spurs' third senior arrival of the summer after fellow defenders Kevin Wimmer and Kieran Trippier.
"I’m very proud to be a Tottenham player," he told the club's official website. "I’m happy to be here and hope to have a wonderful time with this team.
"From the beginning I knew a lot about the club from Jan Vertonghen, Mousa Dembele and Nacer Chadli.
"It’s nice when you join and know a couple of players. I hope to get to know them a little better now!"
Alderweireld's move to Tottenham has been the subject of significant controversy, with Southampton threatening legal action over being denied the use of an option that allowed them to sign the Belgian permanently from Atletico this summer after a successful loan spell at St Mary's last season.