Sky Sports reports that the Magpies are now favourites to sign the 25-year-old defender.
Newcastle are keen to bring in Mariappa as a replacement for Steven Taylor, who will be out of action with an injury for the reest of the season.
Mariappa, who has also been linked with Swansea City, has 18 months left on his current contract with Watford.
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The start of the 2017-18 Premier League season is just around the corner and is shaping up to be one of the most competitive yet.
As always it is the so called “big clubs” that have splashed the most cash in the transfer window so far this summer. Manchester United or City? Arsenal or Tottenham? Who are the winners and losers of the transfer window so far?