The Terriers had not prevailed on their travels since the opening day of the campaign, but a double from Aaron Mooy - either side of goals from Elias Kachunga and Laurent Depoitre - earned David Wagner's team an impressive triumph.
Huddersfield required just six minutes to take the lead through Kachunga, who was able to deliver the final touch to get the ball into the net after a goalmouth scramble.
Kachunga soon had to be withdrawn by Wagner through injury with Tom Ince taking his place, but it did not stop Huddersfield from going further in front when Mooy converted from close range after meeting a cross.
During the early stages of the second half, Collin Quaner headed wide for Huddersfield from close range but it was soon 3-0 when Depoitre found the bottom corner with a well-struck effort.
However, Watford were given hope just after the hour mark when Jonathan Hogg was sent off for a second yellow card and the home side were back in the match when Doucoure drilled a long-range strike into the bottom corner from 25 yards.
The Hornets continued to put Huddersfield under pressure with Richarlison almost getting on the end of a cross from Andre Gray, but Huddersfield remained a threat at the other end and Ince was denied by Gomes.
The points were finally sealed with two minutes left when Huddersfield scored a fourth, with Mooy's spot kick having too much power on it for Gomes to keep it out.
Success for the Yorkshire side has taken them into 11th place in the standings, just a point adrift of Watford who are in ninth position.
Watford (3-4-2-1): Gomes; Mariappa (Pereyra 29'), Prodl, Kabasele; Janmaat, Doucoure, Capoue (Gray 64'), Holebas (Okaka 82'); Carrillo, Richarlison; Deeney
Huddersfield Town (4-5-1): Lossl; Lowe (Malone 45'), Schindler, Zanka, Hadergjonaj; Mooy, Quaner, Hogg, Van La Parra (Williams 64'), Kachunga (Ince 19'); Depoitre