The Clarets’ run of 25 Championship games without defeat — stretching back to late 2015 — came to an end at Vicarage Road.
The visitors once again bossed possession, owning 65% of the ball, but conceded the only goal of the game with what was Watford’s only shot on target.
Tom Cleverley passed the ball into the net right on the stroke of half-time after being picked out inside the penalty area by Ken Sema.
Josh Brownhill almost salvaged a point for the away side with the last kick off the game, but the midfielder’s volley from substitute Vitinho’s cross struck the crossbar.
Here are Chris Boden’s player ratings.
1. Arijanet Muric 6
Didn't have a save to make again, but picked the ball out of his net for a second time this season. Passing again largely excellent, but one slip could have been costly but for Cullen making a block on Sarr. Photo: Richard Heathcote
3. Ian Maatsen 6
Pace down the flank is a huge asset but, aside from teeing up Josh Brownhill and then substitute Luke McNally at the death, his final product was limited. Photo: Richard Heathcote
4. Taylor Harwood-Bellis 7
Happy to roll his sleeves up again, and use of the ball was good. Drove Burnley on after the break, their best player. Photo: CameraSport - Rob Newell