Deulofeu produced a finish similar to his stunning effort at Wembley earlier this month in Watford's FA Cup semi-final win over Wolves to give his side an early lead.
The Spaniard then sealed the points for the Hornets with 10 minutes to go before Huddersfield substitute Karlan Grant headed a stoppage-time consolation.
Watford's sixth win on the road this season kept them on course for their highest Premier League finish, while relegated Huddersfield matched another unwanted record.
The Terriers, who became only the second club after Derby in 2008 to be relegated from the Premier League in March, lost their 14th league game at home this season to match Sunderland's mark set in 2002/03 and 2004/05.
Grant's late header at least lifted Town up to 20 league goals this season, level with Derby's record low of 2007/08.
Huddersfield found themselves a goal down in the fifth minute following an unfortunate error.
Jonathan Hogg slipped after taking goalkeeper Jonas Lossl's pass 20 yards from his penalty area and although, Terence Kongolo's last-ditch tackle denied the surging Abdoulaye Doucoure, Deulofeu struck from the rebound.
The Spaniard showed the same technique he used against Wolves at Wembley, with time to pick his spot.
Huddersfield's best early chance fell to Aaron Mooy, whose free-kick on the left edge of the area arced narrowly wide in the 17th minute.
Town winger Isaac Mbenza, back after illness and making his first start since February, tested Watford goalkeeper Ben Foster with a fierce 20-yard shot.
Huddersfield threatened again soon after. Mbenza and Tommy Smith combined down the right and, from the latter's cross, Elias Kachunga was superbly denied by Adrian Mariappa.
Mariappa, stand-in skipper for Watford's suspended striker Troy Deeney, snuffed out another ball in from Smith, while Huddersfield 18-year-old midfielder Matty Daly made his first-team debut just before half-time in place of Hogg.
Town striker Steve Mounie's overhead kick cleared the crossbar shortly after the restart and, at the other end, Deulofeu forced Lossl into a low save.
Foster comfortably held Mooy's low effort as Huddersfield maintained their game pursuit of a rare Premier League goal, but crosses from Smith and Mbenza eluded everyone in the box.
Deulofeu sealed victory for Watford in the 80th minute when he side-footed home after Lossl had saved Ken Sema's initial shot.
Huddersfield gave their fans something to cheer in the third minute of added time when Grant, a 66th-minute replacement for Mounie, nodded home Chris Lowe's headed pass.