Steve Davies grabbed the only goal as Derby recorded their second successive league win at the expense of Watford.
Davies scored his second of the season when he received the ball from Lee Croft 20 yards out and fired his effort into the roof of the net.
Watford went close early on when Marvin Sordell's effort was well saved by goalkeeper Frank Fielding.
Gavin Massey nearly equalised for the Hornets in the closing stages but but missed with two headers.
The home side will consider themselves unlucky as Derby created very few chances and arrived on the back of a Carling Cup defeat against Shrewsbury.
Sordell had another chance midway through the first half, but his 18-yard effort was easily saved by the busy Fielding.
The Hornets drove forward again five minutes before half time, but Craig Forsyth shot wide after having a clear sight of goal.
Watford tried again after the break but Fielding came to the rescue when he tipped Sordell's 20-yard effort around the post.
Moments later, the Derby keeper was involved once more when he saved John Eustace's header with his feet.
The Rams held on until the final whistle for their first win at Vicarage Road since 2009 and their first away win in seven league games.