Watford lost at home for the fifth time in six matches as Championship strugglers Huddersfield came from behind to win at Vicarage Road.
On his first start since returning to Watford on loan last month, Emmanuel Dennis struck the opener 10 minutes into the second half but Danny Ward came off the bench to score twice in the final 20 minutes.
It meant a winning start for new Terriers boss Andre Breitenreiter but heaped more pressure on under-fire Hornets boss Valerien Ismael.
Watford are just nine points clear of the drop zone and seven points ahead of the Terriers, who climb to 19th.