Mariappa made his Watford debut in 2005

The 20-year-old is a graduate of the youth academy and established himself in the first team this season, making 24 appearances.

"I am very excited to sign for another two years and I'm looking forward to next season, getting out there and playing," he told the club's website.

"Manager Aidy Boothroyd has had the faith in me. He didn't have to put me in the team but he has stuck with me."