Joao Pedro scored the winner on his 19th birthday as Watford made it two wins out of two against fellow early high-flyers Luton Town.
The Hornets striker netted just after the half-hour mark after good work down the left from Ken Sema to continue his side's unbeaten start to the season.
Nathan Jones' visitors, who had won two out of two in the Championship so far, started the game well and hit the bar the first-half when Harry Cornick cut the ball back from James Collins who missed the target from close range.
The Hornets nearly added a second late on when Tom Dele-Bashiru forced Simon Sluga into a good save but Vladmiri Ivic's side held on for another win at Vicarage Road.