Barnsley climbed out of the Championship relegation zone after consigning play-off hopefuls Leicester to their third straight defeat.
The Tykes made the perfect start when Michael Keane bundled Scott Wiseman's fizzing cross into his own goal.
Chris O'Grady steered in the second, while strike partner Jason Scotland missed a second-half penalty.
Nigel Pearson's side have slipped from second to seventh after winning only one of their last 10 outings.
And in just under two months Pearson's side have gone from fighting for the title to outside the play-off places.
Barnsley started the match in the bottom three after Saturday's defeat by fellow strugglers Sheffield Wednesday, but did not look like a side fighting the threat of relegation as they earned their third straight win at Oakwell.
Instead, it was the Foxes who looked like a team battling at the wrong end of the table after a performance which was jeered throughout by the visiting supporters.
Scotland thought he had doubled Barnsley's advantage moments after they were gifted the lead by the hapless Keane, but saw his effort ruled out for offside.
Pearson's men were unable to respond as Barnsley twice went close with further efforts from Jacob Mellis and Martin Cranie.
Recalled midfielder Stephen Dawson, on his return from injury, then found the wrong side of the Leicester post with a delightful lob over Kasper Schmeichel.
But Barnsley deservedly got their reward when former Foxes trainee O'Grady steered a cool finish beyond Schmeichel following a neat interchange with Scotland.
Shortly after the restart, Leicester defender Paul Konchesky clumsily fouled David Perkins inside the penalty area, but Scotland's tame effort down the middle was blocked by the feet of Schmeichel.
Leicester offered little in response until the final stages. Jeffrey Schlupp had a strike ruled out for offside before ex-England striker David Nugent headed over the bar.
Tykes goalkeeper Luke Steele fumbled a looping ball on to his own crossbar but recovered well to block Schlupp's rebound as his team clinched their eighth clean sheet of the campaign.
Barnsley manager David Flitcroft:
"It could have been four, five, six. The two goals, I'd love to see them again. The passing, the intricacy, wow. I wish we were watching them on telly tonight.
"We could have been talking about a huge scoreline because that's how much we dominated Leicester today. We were brilliant.
"The top teams want to play you, they've got great players. Leicester are a team of superb individuals but we didn't let them play today.
"It's an honour to be a manager on a day like this. Our football was just exhilarating."
Leicester manager Nigel Pearson:
"I think he would be quite correct in that. We were way off the mark today, the entirety of the first half was very poor.
"We remain a side that has too many players out of form at the moment. Collectively you can still negotiate these types of games but we're not doing that.
"I don't feel that we are making enough good decisions during the game. It was not a very good performance at all.
"There's no point bemoaning things that don't happen. It's my team and I'll take responsibility for this performance.
"I totally understand the frustration of the fans and dealing with the expectations has been a hindrance at times."
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