Wolves take their place in the relegation zone, with manager Dean Saunders still searching for a first win in charge after seven games.
It looked like tonight might be the night at Oakwell as they led through Bjorn Sigurdarson's cool finish inside the opening 10 minutes and Jamie O'Hara then hit the bar. But they were undone in the second half and are now without a win in 11 games.
They did not show any early trepidation despite their lack of form and they were into a seventh-minute lead courtesy of Sigurdarson.
But it was Bakary Sako's defence-splitting pass that made the goal as the Frenchmen threaded an inch-perfect pass to his team-mate, who rounded Luke Steele and finished into an open net.
They came within inches of doubling the lead when O'Hara was given time and space and curled a wicked 30-yard effort crashing into the underside of the bar, with the naked eye suggesting it bounced over the line.
And while Saunders' men were ruing their luck they were almost caught on the counter-attack as Jim O'Brien set striker Dagnall clear, but he dragged his shot wide.
That missed chance did at least kick-start the Reds into action and Marlon Harewood missed a glaring chance from O'Brien's cross, glancing his header wide of the upright.
It was virtually one-way traffic as Barnsley searched for an equaliser, though in Sako they came up against a clear danger on the break as he twice went close, first blazing over and then testing Steele low down.
Then, after a superb turn, Harewood found himself in a glorious position to get a deserved leveller, but opting not to shoot he rolled the ball across goal and out of play.
The half-time interval did little to stem the tide and eventually the hosts were level when Chris Dagnall atoned for his earlier wastefulness by heading Rory Delap's cross in off the post. It was the former Rochdale striker's seventh goal in eight games and just reward for the Reds.
That early goal set them up for a second-half onslaught, though it never really materialised and Wolves created the better chances with Sylvan Ebanks-Blake curling wide and then Sigurdarson denied a second by a fine save from Steele.
But Barnsley came again and they went ahead with 17 minutes remaining.
Dagnall foraged down the left, centred for O'Grady to set it back for substitute Jacob Mellis, who side-footed a deflected effort into the bottom corner.