Match ends, Watford 2, Derby County 3.
Derby took the lead three times at Watford as they recorded their second successive Championship victory under new manager Steve McClaren.
Jamie Ward's header put the Rams ahead early in the first half, but Fernando Forestieri equalised within minutes.
Ward's tap-in meant Derby led again at half-time, before Lewis McGugan's superb curled shot levelled the scores.
The hosts dominated much of the second half, but could not stop substitute Connor Sammon slotting home late on.
The match started at pace as Watford's Essaid Belkalem headed wide after being unmarked at a corner, before Ward outjumped Daniel Pudil to head home Craig Bryson's cross.
But the visitors had the lead for only four minutes, as neat interplay between Josh McEachran and McGugan put Forestieri through on goal to delicately chip keeper Lee Grant.
McClaren spent much of the match in the upper reaches of Vicarage Road's semi-demolished East Stand, but descended in time to see his side go ahead by the break.
Bryson robbed Iriney and exchanged passes with Chris Martin before sliding the ball across to Ward for an easy second.
Derby almost extended their lead early in the second half with Ward and Bryson going close, but soon found themselves on the back foot as Watford poured forward.
Former Nottingham Forest man McGugan was the recipient of Derby supporters' catcalls as he twice struck free-kicks over the bar.
But the midfielder silenced them with a fantastic strike, curling the ball in from outside the box after trading passes with Pudil.
The introduction of muscular substitute Sammon helped Derby retain the ball in the opposition half, and he clinched all three points for the Rams when Will Hughes sent him through to slot home.
Watford head coach Gianfranco Zola:
"It is tough to take but we have to blame ourselves. It is not enough to play good football and score two great goals.
"We need to get better in defending.
"To win championships, you have to be able to attack well - as we did - but we need to defend better."
Derby manager Steve McClaren:
"We talked about the changes and it was a decision to make, do we put a defender on and try to stop the flow and try to hang on, or change the system and try to go forward?
"[Assistant manager] Paul Simpson was saying, 'go for it, let's try and get the winner'.
"We decided to do that and he did a great job, Connor - it worked."
- 6EkstrandBooked at 79mins
- 5BelkalemBooked at 59mins
- 23Santos Da SilvaBooked at 22minsSubstituted forMurrayat 55'minutes
- 20FabbriniSubstituted forAcuna Caballeroat 76'minutes
- 15Acuna Caballero
- 2A SmithBooked at 78mins
- 3ForsythBooked at 64mins
- 27WhitbreadSubstituted forBuxtonat 33'minutes
- 20BennettSubstituted forDawkinsat 45'minutes
- 10WardSubstituted forSammonat 75'minutes
- James Linington
- Shots on Target
Second Half ends, Watford 2, Derby County 3.
Joshua McEachran (Watford) wins a free kick in the defensive half.
Foul by Craig Bryson (Derby County).
Foul by Gabriele Angella (Watford).
Jake Buxton (Derby County) wins a free kick in the defensive half.
Foul by Marco Cassetti (Watford).
Chris Martin (Derby County) wins a free kick on the left wing.
Corner, Derby County. Conceded by Essaïd Belkalem.
Foul by Essaïd Belkalem (Watford).
Chris Martin (Derby County) wins a free kick in the attacking half.
Essaïd Belkalem (Watford) wins a free kick in the defensive half.
Foul by Conor Sammon (Derby County).
Foul by Essaïd Belkalem (Watford).
Craig Bryson (Derby County) wins a free kick on the left wing.
Goal! Watford 2, Derby County 3. Conor Sammon (Derby County) right footed shot from the centre of the box to the centre of the goal. Assisted by Will Hughes.
Offside, Watford. Gabriele Angella tries a through ball, but Marco Cassetti is caught offside.
Attempt saved. Conor Sammon (Derby County) left footed shot from the left side of the box is saved in the centre of the goal. Assisted by John Eustace.
Attempt missed. Gabriele Angella (Watford) right footed shot from the right side of the box misses to the right.
Corner, Watford. Conceded by Richard Keogh.