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."
Live Text Commentary
Full Time The referee blows for the end of the match.90:00 +3:43
Corner taken short by Chris Dagnall.90:00 +0:21
Matty James challenges Stephen Dawson unfairly and gives away a free kick. Direct free kick taken by Bobby Hassell.88:22
Inswinging corner taken from the right by-line by Paul Konchesky, Luke Steele makes a save.88:07
Liam Moore takes a shot. Luke Steele makes a save.86:06
Direct free kick taken by Bobby Hassell.86:06 Booking
Booking Ben Marshall is booked.85:58
Ben Marshall challenges Stephen Dawson unfairly and gives away a free kick.85:07
Jeff Schlupp takes a shot. Clearance made by Scott Wiseman.84:07
Corner taken by Ben Marshall from the left by-line played to the near post, Luke Steele makes a save.83:51
Jeff Schlupp takes a shot. Luke Steele makes a fantastic save.82:40
Liam Moore produces a right-footed shot from just outside the box that misses to the right of the goal.80:44 Substitution
Substitution Jim O'Brien replaces Jacob Mellis.80:17
Corner taken right-footed by Ben Marshall to the near post, clearance by David Perkins.79:54
David Nugent takes a shot. Brilliant save by Luke Steele.77:43
The referee blows for offside. Free kick taken by Luke Steele.76:53
Outswinging corner taken left-footed by Ben Marshall played to the near post, Kelvin Etuhu makes a clearance.75:49
Bobby Hassell restarts play with the free kick.75:49 Booking
Booking David Nugent receives a yellow card.75:39
David Nugent concedes a free kick for a foul on Stephen Dawson.
David Nugent gives away a free kick for an unfair challenge on Bobby Hassell. Free kick taken by Luke Steele.74:15
Paul Konchesky challenges Chris O'Grady unfairly and gives away a free kick. Free kick taken by Scott Wiseman.71:34
Ben Marshall delivers the ball, David Nugent takes a shot. Save by Luke Steele.70:27
The referee blows for offside. Indirect free kick taken by Kasper Schmeichel.69:17
Matty James gives away a free kick for an unfair challenge on David Perkins. Direct free kick taken by Luke Steele.68:46
Lloyd Dyer fouled by Chris Dagnall, the ref awards a free kick. Paul Konchesky takes the free kick.67:03
Unfair challenge on Lloyd Dyer by Stephen Dawson results in a free kick.67:02
Ben Marshall delivers the ball from the free kick right-footed from right wing, David Perkins manages to make a clearance.67:02 Substitution
Substitution Stephen Foster comes on in place of Martin Cranie.67:02 Booking
Booking Stephen Dawson receives a yellow card for unsporting behaviour.63:23
Paul Konchesky fouled by Chris O'Grady, the ref awards a free kick. Kasper Schmeichel takes the free kick.62:02 Substitution
Substitution Chris Dagnall comes on in place of Jason Scotland.61:03
Corner taken by Ben Marshall from the left by-line to the near post, David Perkins makes a clearance.60:44
Ben Marshall takes a inswinging corner from the left by-line to the near post, clearance made by Bobby Hassell.60:27
Martin Cranie concedes a free kick for a foul on David Nugent. Lloyd Dyer restarts play with the free kick.59:36 Substitution
Substitution Chris Wood goes off and Jeff Schlupp comes on.58:57
Chris Wood takes a shot. Clearance made by Martin Cranie.58:02
Stephen Dawson takes a shot. Kasper Schmeichel makes a save.57:26
Ben Marshall fouled by Stephen Dawson, the ref awards a free kick. Free kick taken by Ben Marshall.55:49 Substitution
Substitution Ben Marshall comes on in place of Anthony Knockaert.55:08
Outswinging corner taken left-footed by Tom Kennedy played to the near post, clearance by Liam Moore.54:41
The official flags David Perkins offside. Kasper Schmeichel restarts play with the free kick.53:51
Inswinging corner taken right-footed by Jacob Mellis played to the near post, Andy King makes a clearance.50:59
The official flags Stephen Dawson offside. Indirect free kick taken by Kasper Schmeichel.50:47 Penalty
A save by Kasper Schmeichel from penalty by Jason Scotland. Barnsley 2-0 Leicester.49:48
Foul by Paul Konchesky on David Perkins, Penalty awarded.47:05
David Perkins concedes a free kick for a foul on Matty James. Paul Konchesky restarts play with the free kick.45:47
Shot from outside the box by Stephen Dawson misses to the left of the goal.45:27
Chris Wood is ruled offside. Bobby Hassell takes the free kick.45:01
The game restarts for the second half.45:00 +2:15 Half time
Half Time The ref blows to signal half-time.45:00 +0:42
Kelvin Etuhu takes a shot. Fantastic save by Kasper Schmeichel.43:50
Free kick awarded for an unfair challenge on Chris O'Grady by Michael Keane. Tom Kennedy takes the direct free kick.41:23
Lloyd Dyer produces a left-footed shot from the edge of the box and misses to the right of the target.39:29
Jacob Mellis provided the assist for the goal.39:29 Goal scored
Goal! - Chris O'Grady - Barnsley 2 - 0 Leicester Goal scored by Chris O'Grady from just inside the penalty area to the bottom right corner of the goal. Barnsley 2-0 Leicester.37:25
Jacob Mellis produces a right-footed shot from just outside the box that misses to the right of the target.36:46
Foul by Anthony Knockaert on David Perkins, free kick awarded. The free kick is swung in left-footed by Tom Kennedy, clearance made by Liam Moore.36:30
Jacob Mellis takes a shot. Blocked by Zak Whitbread.33:51
The assistant referee flags for offside against Liam Moore. Luke Steele restarts play with the free kick.31:20 Substitution
Substitution Liam Moore on for Ritchie De Laet.30:48
Shot from just outside the box by Stephen Dawson misses to the right of the goal.28:20
Unfair challenge on Michael Keane by Jason Scotland results in a free kick. Direct free kick taken by Kasper Schmeichel.27:53
Inswinging corner taken by Jacob Mellis from the left by-line played to the near post, clearance made by Andy King.27:38
Jason Scotland takes a shot. Kasper Schmeichel makes a save.26:51
Anthony Knockaert gives away a free kick for an unfair challenge on Jacob Mellis. Direct free kick taken by Jacob Mellis.26:04
Chris Wood takes a shot. Save made by Luke Steele.22:19
Jacob Mellis takes a shot. Save by Kasper Schmeichel.21:35
Corner taken by Tom Kennedy from the right by-line played to the near post, Andy King manages to make a clearance.20:45
Unfair challenge on Bobby Hassell by David Nugent results in a free kick. Direct free kick taken by Luke Steele.18:17
Free kick awarded for an unfair challenge on Chris O'Grady by Michael Keane. Free kick crossed by Tom Kennedy, clearance by Matty James.17:00
The official flags Stephen Dawson offside. Free kick taken by Kasper Schmeichel.16:20
Lloyd Dyer is caught offside. Luke Steele takes the free kick.14:28
Anthony Knockaert takes a inswinging corner to the near post, clearance by Bobby Hassell.12:52
Zak Whitbread concedes a free kick for a foul on David Perkins. The ball is delivered by Jacob Mellis, Bobby Hassell takes a shot. Kasper Schmeichel makes a save.8:16
Stephen Dawson gives away a free kick for an unfair challenge on Lloyd Dyer. Paul Konchesky takes the free kick.7:30
The official flags Jason Scotland offside. Kasper Schmeichel takes the free kick.5:13
Kelvin Etuhu concedes a free kick for a foul on Chris Wood. Paul Konchesky produces a shot on goal direct from the free kick.4:43
Chris O'Grady gives away a free kick for an unfair challenge on Zak Whitbread. Kasper Schmeichel takes the direct free kick.3:44
Assist by Scott Wiseman.3:44 Goal scored
Goal! - Michael Keane - Barnsley 1 - 0 Leicester Michael Keane puts the ball into his own net. Barnsley 1-0 Leicester.0:00
The referee starts the match.
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