The Belgium international has at last completed his move from Ajax to the Copa del Rey winner, which has also signed Josuha Guilavogui.
The Champions League hopeful was keen to strengthen the defense before the end of the summer transfer window and has now finalized the transfer of Alderweireld on deadline day to do so. The Belgium international, whose Ajax deal was due to expire in 2014, had already passed his medical on Saturday.
Alderweireld, 24, joined the Ajax youth academy from Germinal Beerschot in 2004 and had developed into a key player with the Eredivisie champion in the past few seasons. He had also caught the attention of clubs such as Bayer Leverkusen and Liverpool this summer, but will now continue his career at Atletico instead.
"I wanted to sign for one of the best club's in Europe," Alderweireld told the club's official website. "I'm happy to be a part of this club."
Atleti has also signed Saint-Etienne midfielder Josuha Guilavogui from Saint-Etienne for a fee believed to be in the region of 10 million euros.
The France international, 22, has spent all of his professional career at Stade Geoffroy-Guichard, but has been widely linked with a move away from the club this summer. Diego Simeone's side saw off reported competition from the likes of Arsenal, clinching his signing on a five-year deal.
"I am pleased to have moved to a club as big as Atletico and to pay for these fantastic fans. I hope to work hard so that the team continues to win titles in the coming years," Guilavogui said.