Two goals from Grzegorz Rasiak helped Watford to a victory that was not quite as emphatic as the scoreline suggests.
Barry Bannan hit the bar from 20 yards for the visitors before Jobi McAnuff prodded Watford ahead six yards out.
Tamas Priskin headed a free-kick against the bar but Rasiak converted the rebound and then added a second from 12 yards after Tommy Smith's pass.
Derby had the better of the second half but could only manage one goal, John Eustace heading in Gary Teale's cross.
Watford manager Brendan Rodgers:
"It was good to see the motivation of the players.
"I said a few months ago that our season will not finish until the last whistle goes against Derby and the players have made me proud.
"We have made progress this year, my job next year is to make real progress. I didn't come to be comfortable, I want to enjoy the pressure and some of me didn't enjoy today because there was no real pressure."
Derby boss Nigel Clough:
"I thought we created enough chances and if we had got the first goal we might have run out 3-1 winners.
"But if we are going to be giving goals away like that we will not get anything out of the game."
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