The Netherlands right back is set to replace Arsenal-bound Mathieu Debuchy at St. James' Park.
The 24-year-old will replace Mathieu Debuchy at right back for the Magpies, meaning the Frenchman can complete his move to Arsenal.
Janmaat, who is set to sign a six-year deal at St. James' Park, will become the club's fifth signing of the summer. He is unlikely to make the preseason trip to New Zealand, however, given he has only just returned to Europe following his participation in the World Cup.
After initially seeing its first bid rejected last week, Newcastle agreed to a deal worth around 8 million euros with Feyenoord.
Janmaat should become the club's second signing in 48 hours after Remy Cabella joined from Montpellier for around 15 million euros Sunday.