The 26-year-old Belgium international defender joined Spurs this week despite the Saints already having a deal in place to sign him as part of last-season’s loan.
The south coast club are currently considering legal action against Atletico for failing to honour their agreement but Alderweireld says the lure of a new challenge with Belgium team-mates Jan Vertonghen and Nacer Chadli at White Hart Lane was too attractive to turn down.
“I understand that there is some disappointment,” he said. “I have sent a text message to Ronald and thanked him for a great year.
“He gave me a lot of confidence and made me a better footballer. Thanks to Southampton I am able to be on the map again after a difficult year at Atletico.
“I gave 100 per cent for Southampton and wish everyone luck for next season, but I have always been closely following Tottenham. It’s always been a dream to play for a very big club this size and now it’s come true. I’m very proud.
“I got the impression that Spurs would like my help. Their stats were the reason why the club wanted to reinforce defensively. It’s good that I have a year of experience playing in the Premier League.
“But also I joined because there are so many Belgians. It will be fun to be with them every day on the training ground.
“Playing centre-back, right-back or in midfield, I will always give my all for this club. I’m looking forward to showing myself for the fans.”