The 22-year-old has become a regular at right-back under manager Brendan Rodgers and was due to leave this summer when his contract expired but is now looking forward to improving for the Hornets.
Rodgers said: "Adrian is someone I think who will grow over the next few years because of how we want to play.
"You'll start to see things in him that you may not have seen before. I've enjoyed working with him because he's a learner.
"I'm not interested with having players who think they know everything but he is the opposite. He loves Watford and loves being here."