Andy Ritchie's side cruised to a 2-0 success over Forest, who had been the pre-season title favourites.
It had only been Forest's fine form at the City Ground that had kept them on the shirt tails of the promotion pack, following a run of ten straight away games without a win.
But, for the first time since early September, when Brentford triumphed on the banks of the River Trent, Forest lost at home, and could have few complaints.
One of the low points of Forest's season had been provided by Barnsley, with a 2-0 defeat at Oakwell in September that was enough to prompt Gary Megson to offer his resignation.
But, given their home form, few would have expected the visitors to record a double over the summer title favourites.
That seemed the likely outcome, however, as Ritchie's side made the brighter start and seemed early on to have found the key to unlock the Forest defence.
Three times in the first 15 minutes, Barnsley were able to find gaps in the home defence and it was no surprise when the visitors went ahead.
Paul Hayes had already fired a warning across the Forest bows, with a driven low shot that Paul Gerrard was well placed to save.
But it was not heeded by Forest who, within a matter of moments, were caught napping again, this time as a Hayes provided the flick on from a Matt Carbon ball from defence, to send Marc Richards racing clear.
And the striker made no mistake, drawing Gerrard off his line before firing a powerful shot that rose over the goalkeeper and into the back of the net.
It could have been worse for Forest, as Chris Shuker broke clear down the right and angled into the box, before striking a rising effort that hit the side netting.
Nathan Tyson, as ever, was Forest's most dangerous threat. After striking an effort just wide prior to the Barnsley goal the striker then broke clear of the visiting defence, with Paul Reid failing in an attempt to bring him down, but the former Wycombe man fired wildly over the bar.
But it was a mistake at the other end that was to be instrumental in Barnsley adding a second, as Wes Morgan lingered too long in possession and allowed Richards to rob him and charge towards goal for a second time.
It produced the same result, as the striker again provided a sublime finish, sending a dipping shot over Gerrard and into the back of the net.
It could have been even worse for Forest, as Shuker twice came close to adding a third, with one effort in particular flying only inches wide of the post after the home defence again proved fragile.
Forest provided sterner resistance after the break, with Grant Holt forcing a strong save from keeper Nick Colgan and Tyson again coming close with a rising shot.
But it was Barnsley who remained in control and who looked the more likely to score, with Neil Austin only being denied by the bar.