The 24-year-old right-back has signed a six-year contract and will be a direct replacement for Mathieu Debuchy, who has been sold to Arsenal.
Janmaat arrives fresh from the World Cup in Brazil, where he made five appearances as the Netherlands made it through to the semi-finals before losing on penalties to Argentina.
Having begun his career with ADO Den Haag, he joined Heerenveen in 2008 and made over 100 appearances there before joining Feyenoord in 2012.
Janmaat is excited about his move to the Premier League and told Newcastle's official website: “This is a fantastic club, with a big history and a great stadium. 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."
Magpies boss 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."