The 24-year-old Netherlands defender is expected to sign for the Magpies, who have already agreed a deal with Feyenoord - although details of the transfer fee have yet to be disclosed.
It appeared an announcement was imminent with a photograph of the player holding a back and white shirt alongside Magpies manager Alan Pardew appearing on Twitter later on Tuesday.
However, some minor formalities remain to be completed and that means Janmaat will not now be unveiled at St James' Park until Wednesday.
Should Janmaat join Newcastle, he will not be heading to New Zealand later this week for the club’s pre-season tour, as he will be taking a well-earned break following his participation at the recent World Cup finals in Brazil.
Janmaat's arrival would allow Mathieu Debuchy to move to Arsenal.
Gunners boss Arsene Wenger is keen to buy the France right-back but Newcastle are holding firm until their replacement signs on the dotted line.
Meanwhile, manager Alan Pardew is also hoping to secure Monaco striker Emmanuel Riviere for a reported fee of £6m.