Conor Hourihane hit a superb 88th-minute winner to maintain Barnsley's play-off push and add to Nottingham Forest manager Philippe Montanier' worries.
Nicklas Bendtner had a goal ruled out for offside for Forest seconds before Hourihane rifled home a shot from 30 yards to earn the Tykes a 1-0 win at the City Ground As Paul Heckingbottom's Barnsley celebrated a fifth win in seven games, under-pressure Montanier was left to contemplate a fifth defeat in six outings.
The Frenchman was hoping for a victory to bolster his position, with an American consortium set to complete a takeover of the club, but faced only more frustration following a game his side could easily have won.
Eric Lichaj lifted a speculative effort over the bar early on for Forest, but it was the recalled home goalkeeper Stephen Henderson who was the first to be drawn into meaningful action, as he got down well to his right to push away a bending, dipping free-kick from Adam Hammill.
From the resulting corner, Barnsley should have taken the lead when Josh Scowen was picked out unmarked at the far post, but he lifted a first time effort over from close range.
Forest might have netted themselves from a corner when Ben Osborn's curling delivery picked out Britt Assombalonga in the centre, but his powerful header flashed wide.
Another Barnsley corner saw Scowen threaten again, with Henderson making a superb save to block after the Barnsley man had found room to unleash a powerful shot from a tight angle eight yards out.
Forest almost took the lead when Assombalonga's powerful low cross-shot caused chaos in the Barnsley box, with Andy Yiadom's panicked clearance only flying wide by inches.
The second half saw both Lichaj and Thomas Lam come close as Forest started brightly before Jack Hobbs powered a header against the bar from a Ben Osborn corner.
Substitute Apostolos Vellios might have broken the deadlock when he was picked out by a deep cross from Osborn, but the Greek made a poor connection at the far post, sending his shot high.
The previous clash between these two sides had ended in a 5-2 win for Forest at Oakwell, but both sides were struggling to make a breakthrough here.
It was the unlikely figure of Lichaj who almost broke the deadlock, with the Forest defender, who has scored only two career goals for Forest, turning neatly on the edge of the area to make room for an effort that flew narrowly off target.
Forest did have the ball in the back of the net in the 86th minute when sub Bendtner turned and beat keeper Adam Davies after being fed by Osborn, but the effort was ruled out for a close offside decision.
Instead it was Barnsley who snatched the points in spectacular fashion.
There seemed to be little danger when Hourihane collected the ball 30 yards out, but the Tykes captain had other ideas, taking a touch before rifling an unstoppable shot high into the net.
Matty Cash was close to snatching a point for the home side, but glanced an injur- time effort wide.