Tommy Smith fired a dramatic late equaliser to heap more pressure on boss Simon Davey and leave Barnsley fearing for their Championship future.
With the Tykes looking set for a priceless victory, Smith popped up with two minutes left at Vicarage Road to grab a deserved equaliser and keep Davey's men firmly in the relegation mire.
Jon Macken's goal on 67 minutes looked to have won the day, but Smith was on hand to ensure the Yorkshire side are still battling grimly for survival.
A draw looked an unlikely result given Barnsley's dreadful first-half display, and it was only good goalkeeping and the woodwork that kept them on level terms.
Watford had picked up the momentum after a slow start and Tamas Priskin came within a whisker of scoring when his 20-yard rocket struck a post with Heinz Muller beaten.
After Davey's pitchside bust-up with Sheffield United boss Kevin Blackwell last week, the Barnsley manager would have hoped for similar passion from his players.
But they looked as if they would rather have been anywhere other than fighting for survival against a determined and physical Watford. Yet the Tykes almost pinched the lead on 24 minutes when Bobby Hassell's scuffed cross looped over Scott Loach before dropping on to the bar.
After their flat first-half show, Barnsley emerged in determined mood and only a point-blank save by Loach prevented Michael Mifsud from scoring.
The Tykes continued to look the more dangerous, but Davey's decision to replace Daniel Bogdanovic with midfielder Adam Hammill led to chants of "You don't know what you're doing" from the travelling support.
But the same fans were soon singing a different song as Mifsud's pinpoint cross picked out Macken inside the penalty area, allowing the striker to smash home from close range. Substitute
Kayode Odejayi should have added a second with five minutes remaining, but his header was brilliantly turned away by Loach.
That turned out to be a huge moment in the contest.
With 88 minutes on the clock, Watford star Smith was on hand to stab in Adrian Mariappa's cross and cruelly snatch away Barnsley's precious win.
Watford: Loach 7 - Mariappa 7, DeMerit 6, Williamson 6, Doyley 6 - McAnuff 7, Cork 6, Jenkins 6, Cauna 5 (Rose, 49mins, 6) - Priskin 7 (Hoskins, 63mins, 6), *SMITH 7.
Barnsley: *MULLER 7 - Kozluk 6, Moore 6, Foster 7, Hassell 6 - Campbell-Ryce 7, Da Silva 6, Teymourian 6 Bogdanovic 6 (Hammill, 56mins, 7) - Mifsud 6 (Odejayi, 83mins), Macken 6 (Colace, 87mins).
Referee: D Whitestone 5.