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 reputation of the Premier League means it is the most popular and one of the most watched leagues in the world.
Five English clubs will compete in the last sixteen of the Champions League for the first time ever as Liverpool, Manchester United, Manchester City, Chelsea and Tottenham face some difficult away matches.