Tom Cleverley's winner helped 10-man Watford extend their good start to the Championship season and end Burnley's unbeaten start under Vincent Kompany.
The hosts led with the game's first effort on target just before half-time as Joao Pedro's neat touch freed Ismaila Sarr, whose pull-back found Cleverley to side-foot into the bottom corner.
Josh Brownhill was twice denied by smart saves from Hornets goalkeeper Daniel Bachmann as the Clarets improved after the break.
The hosts lost full-back Hassane Kamara to a straight red card for pulling back Vitinho as he bore down on goal, and Brownhill then turned the Brazilian's cross against the bar, but Watford held out.