The Spanish forward profited from some slack Huddersfield play early on to open the scoring at John Smith's Stadium and converted a rebound from close range late on to end the Hornets' three-match losing run on their travels.
Watford had lost three of its past four top-flight matches, either side of beating Wolverhampton Wanderers to book a place in the FA Cup final, but it got back on track in the west Yorkshire sunshine despite Karlan Grant's late consolation.
Leicester City's draw at West Ham United and two dropped points for Wolves at home to Brighton and Hove Albion lifted Watford into seventh place, while Huddersfield's 14th home loss of the season — equalling the Premier League's worst record set by Sunderland in 2002-2003 and 2005-2006 — sees the Terriers cut further adrift at the bottom with their relegation fate already confirmed.
Despite making five changes to the side that lost 4-0 at Tottenham Hotspur last weekend, it was a familiar story for Jan Siewert's men as they were behind inside the opening five minutes.
Jonathan Hogg was caught in possession and Abdoulaye Doucoure picked out Deulofeu, brought into the side for the suspended Troy Deeney, to convert with a dinked finish via the inside of the post.
Only a save from Jonas Lossl prevented Ken Sema from doubling his side's tally but Huddersfield, which brought on teenage midfielder Matty Daly for a debut with 41 minutes of the match played, also came close through Isaac Mbenza's well-struck attempt that forced Ben Foster to scamper across his goal.
Huddersfield improved as the match went on and nearly levelled when Steve Mounie hooked a shot over the bar, shortly before an off-balance Aaron Mooy was denied by Foster.
Deulofeu added his second goal of the match 10 minutes from time when slotting a rebound past Lossl, who palmed Isaac Success' shot into the ex-Barcelona star's path, rendering substitute Grant's fourth top-flight goal of the season — a header inside the six-yard box in added time — a mere consolation.