Hassell, on as a sixth-minute replacement for the injured Rory Delap, headed home Jacob Mellis' cross 10 minutes before half-time.
It was the defender's first league goal since scoring for the Tykes in a 1-1 home draw with Swansea on January 22, 2011.
After climbing out of the bottom three with victory over Brighton at Oakwell on Tuesday night, Barnsley are now two places and two points above the drop zone.
With 39 away league goals before Saturday, Watford are by far the Championship's highest scorers on their travels but they lacked their usual attacking flair against the Tykes.
The Hornets stay third in the table though, for now, ahead of Crystal Palace's trip to Brighton on Sunday, and remain two points behind second-placed Hull, who lost at home to resurgent Nottingham Forest, following back-to-back defeats.
Barnsley goalkeeper Luke Steele was called into action early on to push away Jonathan Hogg's header and Matej Vydra blasted a 20-yard shot way over the crossbar as the half-hour approached.
The hosts had their first real effort on goal after 33 minutes as Jason Scotland forced a good save from Jonathan Bond.
Two minutes later, the Tykes took the lead. Mellis got free down the left and whipped in a pacy cross which Hassell guided into the bottom left-hand corner of the net from 12 yards out.
Watford were unable to respond in a frantic final 10 minutes of the first half as Barnsley defended stubbornly.
Chris O'Grady almost made it 2-0 at the start of the second half but Hogg got down quickly at his near post to save.
Vydra finally got a clear sight of goal in the 73rd minute but his volley produced a good save from Steele who held on to the shot.
David Flitcroft's side continued to stifle the visitors while creating chances of their own.
Half-time substitute Chris Dagnall, on for Scotland, cleared the crossbar with a shot after 84 minutes before O'Grady shot straight at Bond with three minutes of the regulation 90 remaining.