In a game low on quality and chances, Pedro conjured a moment of inspiration with Watford’s first serious attempt on goal to settle the contest.
Wayne Rooney had a good chance to snatch a point but shot wide and Derby have now lost four of their opening five matches, while Watford are up to fifth.
Watford had scored only two goals in four league games this season but they almost caught Derby out with a long ball over the top in the 12th minute, only for David Marshall to get there before Ken Sema.
Rooney scored a sublime free kick at Norwich before the international break and his delivery from a corner caused the first moment of concern for Watford, although no white shirt was able to take advantage as the ball sailed across the six-yard box.
Both sides created little from open play with too many passes misplaced but Derby had a chance in the 40th minute to take the lead.
A loose pass from Tom Cleverley presented Jason Knight with possession and he quickly played in Rooney, who picked out Duane Holmes only for the former Scunthorpe midfielder to fire over from 15 yards.
There was little to suggest at the start of the second half that the game was going to open up, although Watford had strong claims for a penalty waved aside in the 60th minute when a Nathaniel Chalobah shot appeared to strike George Evans on the arm.
Derby brought on Tom Lawrence for his first appearance of the season to try and inject some creativity and the Wales international had his side’s first shot on target with a low drive straight at Ben Foster in the 74th minute.
But two minutes later, Watford struck with their first effort when Pedro curled a shot from the left edge of the area into the top corner to give Marshall no chance.
Derby had a chance to respond immediately when Lawrence played Rooney in but he shot wide when he should have at least tested Foster.
Lawrence certainly made a difference and he had another effort blocked as Derby pressed hard, while Rooney had a stoppage-time free-kick dip just over the bar on another disappointing night for the home side.