Gerard Deulofeu scored both of the Hornets goals, the first coming after only five minutes when he curled his shot into the bottom corner of the net after Jonathan Hogg had lost possession inside his own half.
Watford almost doubled their lead in the 11th minute but Jonas Lossl saved Sweden international Ken Semas powerful drive and, when the Terriers finally managed to launch an attack, Juninho Bacunas shot flew over the bar.
As the home team began to get a foothold in the match, Aaron Mooys free-kick was only just off target and Ben Foster made a smart save to deny Isaac Mbenza. Tommy Smith then played in Elias Kachunga but his shot was blocked by Adrian Mariappa.
The second half started slowly until Town goalkeeper Lossl saved Deulofeus speculative effort and Huddersfield striker Steve Mounies overhead kick flew just over the bar.
As the Terriers began to get on top, Foster had to react well to keep Mooys shot out, while at the other end Deulofeu lifted his 73rd-minute shot over the bar with just Lossl to beat.
However, the Spaniard did make it 2-0 seven minutes later when he lashed the rebound home after Lossl had saved Semas effort.
Javi Gracias team conceded a goal in stoppage time when Huddersfield substitute Karlan Grant scored from close range although it was only a consolation for the West Yorkshire outfit.