Alderweireld joins on a season-long loan from Spanish champions Atlético Madrid, where he lifted the La Liga trophy and played in the UEFA Champions League final last season.
The 25 year-old, who can play at centre-half and right-back, has 37 caps for his country. Prior to moving to Atlético last summer, he spent nine years with Dutch giants Ajax, where he won three Eredivisie titles and one Dutch cup.
Ronald Koeman commented: “I am delighted that we have been able to bring Toby to the club.
“I know him very well because he played for some years for Ajax in Holland and he then made a transfer to Atlético Madrid, where he hasn’t been playing an awful lot, which is why he was looking for somewhere else to play.
“I think our first conversation was four or five days ago and I told him about our plans, about the ambition of the club and about how we play.
“He is a strong defender, and is also very good at building up the play when we play out from the back.
“He played in the top league in Spain and against top teams in Europe, so he has a lot of confidence and experience, and he has a big future because he is still young.
“We needed competition in our squad, and with Toby here we now have a lot of possibilities and quality.”
Alderweireld said of the move: “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.”