Janmaat's two-year spell in the north-east of England looks to be about to end as the Dutchman is verging on a move to the Hornets.
The defender is understood to be travelling to Watford for a medical and is expected to sign in the next few days.
Newcastle look to have already found a replacement in Spurs' DeAndre Yedlin and are happy to let the Netherlands international return to the Premier League.
Janmaat has played 77 games for the Magpies over two seasons, scoring 4 goals and also has 27 caps for his country.