Barnsley striker Kayode Odejayi put his FA Cup semi-final miss behind him to score twice at Watford and lift the Tykes out of the relegation places.
Watford - aiming for top spot in the Championship - were poor and Odejayi blazed over before he tapped in from close range after Jon Macken's cross.
Stephen Foster powerfully headed home a second from Diego Leon's corner.
Nathan Ellington had a header cleared off the line for Watford before Odejayi lifted over Richard Lee for a third.
Watford manager Aidy Boothroyd:
"If you can't score from three yards or defend set-pieces you don't deserve to win.
"I'm very disappointed and I would like a few answers from my players tomorrow.
"We win and lose together. We are one of the favourites and when teams come here we have to try and break them down, but we just didn't do it."
Barnsley boss Simon Davey on Odejayi:
"We always believe in him.
"Strikers live and die by their goal ratio and Kay's has not been good. He knows that but he works very hard.
"To bounce back from Sunday like that is incredible."
Watford: Lee, Doyley, Shittu, Sadler (Kabba 56), Bromby, McAnuff, Williamson, O'Toole, Stewart, Henderson (DeMerit 71), Ellington (Priskin 55).
Subs Not Used: Poom, Mariappa.
Barnsley: Steele, Foster, Souza, Nyatanga, Van Homoet, Campbell-Ryce (Togwell 90), Hassell, Leon, Kozluk, Odejayi (Ferenczi 86), Macken (Coulson 90).
Subs Not Used: Letheren, Butterfield.
Goals: Odejayi 36, Foster 47, Odejayi 54.
Att: 16,129
Ref: Paul Armstrong (Berkshire).
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