The 24-year-old, who featured for the Netherlands during their World Cup campaign in Brazil, has sealed a six-year contract.
"Newcastle is a fantastic club, with a big history and a great stadium," Janmaat told the club's official website. "I have also been told all about how great the fans are.
"This is the perfect move for me, and I am really looking forward to being with the team and getting started here at Newcastle. I think we are going to have a great season."
Manager Alan Pardew added: "We are delighted to bring Daryl to the club. He is the perfect example of a modern full-back - someone who is good defensively but offers a real threat going forward as well. Daryl had an excellent World Cup and we are looking forward to him continuing that form for Newcastle."