The 23-year-old Dutch international, who describes himself as a strong, physical central defender, has joined the Saints on a five-year deal for a fee believed to be in the region of £15 million.
"I'm really happy. It has been a long week but I'm happy that everything is finished and I can start my period here at Southampton," he said.
"I think Southampton is a really big club so the next step for me is to come here and play in the Premier League and try to help in the best way that I can.
"The Premier League is the biggest competition in the world. I made my first step to Italy and I had a very good two years over there, but it was always the plan to play in the Premier League."
He added: "I'm a strong, physical central defender with qualities on the ball. My main task will be defending but hopefully I can add those qualities on the ball to Southampton.
"I want to grow alongside the club so I have my goals but the first thing that is important is to start playing, adapt to the league and we will see how far we can go."