Table-topping Plymouth crashed to their first defeat in 11 games as Barnsley picked up their first win under manager Paul Hart.
The win came thanks to a later winner from new boy Gary Birch, on loan from Walsall until the end of the season.
Plymouth might have been in front at half time despite a sparkling start from Barnsley when the visitors, unbeaten in their previous eight games, were twice denied by the woodwork.
Seconds before the break Ian Stonebridge's 22-yard shot struck the upright before Paul Connolly darted in at the near post to fire in a ten-yard shot after David Friio slipped him a neat pass.
But after the break Barnsley, without a goal in their last four games, were eager to give Hart his first victory in five games at the helm.
A 20-yarder from Alex Neil was well saved by keeper Luke McCormick who then made another smart stop to cut out David Murphy's low angled shot.
The winner came nine minutes from the end as a result of Barnsley's tenth corner.
Murphy curled it into the area and Birch rose above the rest to power in a header and put the home side in front.
But in the dying minutes Plymouth came close to an equaliser when Graham Coughlan and Stonebridge tried to stab in close-range shots as the home side panicked but a last-ditch challenge from Neil Austin denied them the equaliser.
Afterwards Tykes boss Paul Hart said: "I'm delighted with the victory. We deserved the win for the performances we have put in for the last five games.
"I was pleased for Gary that he scored - it is always good for strikers to score.
"I was pleased that we won. Plymouth are the top team and the best team in this division and you can see why.
"They have had the same team together for two years and they are very difficult to play against."