Josh Doherty has become the third Hornets Academy player to sign a professional deal at the club in the past week.
The Northern Ireland Under-19 international centre-back has signed a one-year contract at Watford, with the club having the option of an additional 12 months.
The 18-year-old made his Hornets debut on the final day of the season. The young defender replaced Cristian Battocchio with ten minutes remaining of the 4-1 defeat at home to Huddersfield Town.
Watford head coach Beppe Sannino was asked after that defeat about Doherty and said: “It’s a good reward for the lads and the people that work every day behind me.
"Today, when I played Doherty, the game was running down and I thought he could have a good moment on the pitch. I am pleased with him because he did very well.”
Doherty follows Alex Jakubiak and George Byers in signing professional contracts with Watford.