Huddersfield started the game well but struggled to carve out any clear-cut opportunities, with Nahki Wells producing the best effort after 13 minutes, but his shot was straight at Jonathan Bond.
The visitors continued to press and Paul Dixon should have done better when he fired an effort over the bar, while Troy Deeney got back in defence to block a shot from Adam Clayton to keep his side level.
Watford had the best chance of the half when Cristian Battocchio found himself through on goal but he couldn't keep his composure and fired a shot straight at Alex Smithies.
After the break, Huddersfield took the lead as Joe Lolley took advantage of a mistake from Gabriele Angella and was able to slot the ball past Bond.
The Terriers doubled their lead in the 55th minute when Lolley turned provider as he found Ward on the edge of the box, and his low shot found its way past Bond and into the net.
Huddersfield got their third goal as Lolley and Ward combined again, with Ward getting his second goal for the game with a good finish from close range.
Ward completed his hat-trick and the rout in the 87th minute when he guided the ball past Bond, while Deeney got a consolation goal from the penalty spot for the home side in the 90th minute.