Barnsley won at Elland Road for the first time since 1990 with one of their best performances of the season to add to Leeds' troubles at home.
The visitors went ahead when Jim O'Brien squared for Ricardo Vaz Te, whose scuffed shot looped in.
It was 2-0 when Jacob Butterfield's strike was pushed by Alex McCarthy into the path of Craig Davies, who finished.
Leeds' Ross McCormack fired in a 25-yard free-kick, but Barnsley hung on despite David Perkins' second booking.
Robert Snodgrass had an early run and shot for Leeds, but that was the sum of his side's early chances in spite of their possession.
Buoyed by riding the early storm, Barnsley settled into things and their passing - particularly from the outstanding Butterfield - was impressive.
Captain Butterfield ghosted into the box unmarked and should have done better with a free header, while at the other end, Aidy White was booked for taking a dive over the flailing leg of O'Brien in the box.
The same two players were then heavily involved in Vaz Te's 27th-minute goal, although it was another Leeds player taking a tumble this time.
Lloyd Sam slipped as he tracked a Bobby Hassell throw, allowing O'Brien to get in behind White and square for the waiting Vaz Te to score beyond McCarthy.
The keeper then made an error that allowed Barnsley to double their lead two minutes before the break. Even though McCarthy got his body behind Butterfield's shot, he could only shovel it into the path of the on-rushing Davies, who made no mistake.
Leeds looked to up the ante after the break and Snodgrass hit the post with a curling effort that had Luke Steele beaten.
When McCormack slammed home an excellent free-kick with 35 minutes left, after a foul on Luciano Becchio, the atmosphere grew.
United boss Simon Grayson threw on the lively Ramon Nunez in a bid to capitalise on the momentum, but some resolute defending from the Tykes allowed them to stabilise and, when McCormack was withdrawn, they had ridden out the worst of the pressure.
They could then have restored their two-goal cushion had debutant Nile Ranger got on the end of O'Brien's fizzing cross-shot, while McCarthy was fortunate to see White on hand to clear after he shelled another shot from distance, this time from Matt Done.
Perkins was then sent off, but goalkeeper Steele ensured his dismissal was not costly when he saved in added time from Nunez.