The Dutch defender became Alan Pardew's sixth signing of the close-season on Thursday - serving as a direct replacement for Mathieu Debuchy who left for Arsenal on the same day.
After helping Netherlands to a third-place finish at the World Cup, former Feyenoord man Janmaat said he had options in Italy.
However, the 24-year-old insisted that a move to Newcastle was ideal for him at this stage in his career.
"There was some interest from Italian clubs," he is quoted as saying by the Newcastle Evening Chronicle.
"But when I heard from Newcastle my feeling was it was perfect.
"It is a fantastic day for me. When I saw the stadium there were a lot of things here, I'm happy.
"It is a really nice stadium. I've heard a lot from [Netherlands and Newcastle goalkeeper] Tim Krul. I'm really impressed. The fans are also fantastic.
"I'm looking forward to playing here."
