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."
'People will remember us because we got to No.1 in the world rankings and achieved great things - but our biggest regret is that we didn’t win anything': Ex-Everton star Kevin Mirallas opens up on 'honour' of being part of nation's golden generation
‘I’m sure we’d have finished in the top four or five if he hadn’t joined Chelsea in January. Him leaving damaged the team, but we couldn’t stand in his way’: Alan Curbishley reflects on failed Champions League dream after selling key player
