Alderweireld was top of Newcastle's wanted list but Southampton won the battle.
Alderweireld: "I'm very happy to be here.
"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.
"I think this is an upcoming club. Southampton put a lot of effort in to get me, so now I want to give something back to the club by doing my best to help the team. I want to give everything for Southampton."