The home side looked bright from the start and took the lead just after the half-hour mark when a free-kick deflected to Eustace on the edge of the box and he fired a fine effort in off the left-hand post.
Jimmy McNulty spurned a good chance to draw Barnsley level in the 65th minute and three minutes later Eustace burst into the Barnsley box before doubling Watford's lead.
Defender Scott Golbourne marked his Tykes debut with a goal to set up a tense finale but Watford held on for a second consecutive win which moved them 12 points clear of the bottom three.
Watford started well and Joe Garner and Troy Deeney linked up before the latter saw a shot charged down. Moments later Garner miskicked when well placed but the ball fell nicely for Eustace who curled a shot from 20 yards just wide.
The Hornets continued to dominate and Barnsley goalkeeper Luke Steele made a good save down at his near post to keep out an effort from Alex Kacaniklic.
With 32 minutes gone Watford deservedly went in front when Sean Murray's free-kick cannoned off the Barnsley wall into the path of Eustace who adjusted his body well and acrobatically hooked the ball across Steele and in off the post.
Watford immediately went in search of a second and a quick break ended with a Lloyd Doyley shot which was blocked well by Stephen Foster.
Having seen his side produce a toothless first 45 minutes, Barnsley manager Keith Hill brought striker Chris Dagnall on for Matty Done at the break.
Barnsley did eventually start to ask some questions of the Watford defence and Andy Gray headed wide before a dangerous free-kick from McNulty was pushed over the bar by Scott Loach.
Soon after, a free-kick from David Perkins found McNulty unmarked at the far post but he headed over when he should have done better.
Just as Barnsley were looking dangerous, Watford doubled their lead.
A poor header out from Foster fell to Eustace just outside the box and he rode a couple of half-hearted challenges before slipping the ball past Steele.
Steele made a good save to prevent Doyley making it 3-0 and with five minutes remaining Scott Wiseman's cross was flicked on by Craig Davies and Golbourne arrived at the far post to halve the deficit.
Barnsley pressed in the final stages but were unable to find a way through.