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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Summer 2016 is set to be a very exciting time for football fans, as this is when the long awaited Euro 2016 tournament kicks off.
Since Guus Hiddink arrived at the helm, Chelsea have improved considerably after a tricky start to the campaign.