Two goals in two second-half minutes saw Barnsley condemn Watford to defeat.
The Tykes could have been 2-0 up after 20 minutes but it was Watford who went ahead as Jobi McAnuff's cross was converted by Tommy Smith six yards out.
Smith fired over before substitute Miguel Mostto levelled with a clinical finish from just inside the area.
Skipper Stephen Foster hit the winner, pouncing on a loose ball for his first goal of the season, while Tykes keeper Heinz Muller made a key save late on.
Barnsley Manager Simon Davey:
"It's been a hard, trying week for everyone at the club and it's fitting that we won.
"We had plenty of chances and I think we deserved it.
"It's been a long road for Miguel because he's taken time to adjust but he's stuck to his task, worked hard in training and taken his goal well."
BBC Audio Link: Simon Davey