Connor Wickham's first goal for Sheffield Wednesday dented Leicester's Championship promotion chances and boosted his own side's survival hopes.
The 19-year-old striker, on loan from Sunderland, fired in a close-range volley as Wednesday secured only their second win in six league matches.
Defeat puts more pressure on Foxes boss Nigel Pearson, whose side have won only one of their last seven league matches.
The Owls are 19th, four points clear of the relegation zone.
The hosts had started brightly, with striker David Nugent forcing an early save from Wednesday goalkeeper Chris Kirkland with a dipping 20-yard shot.
The former Portsmouth striker sent another effort from outside the area wide before the visitors responded with their first chance in the 12th minute, Leroy Lita flashing a header wide.
But Kirkland remained the busier of the two keepers as Ben Marshall and Ritchie De Laet tested him before half-time.
However, when Wickham came off the bench just before the hour mark, the Foxes were made to pay for their wasted opportunities, the former Ipswich striker crashing a close-range volley into the roof of the net from Lewis Buxton's cross to send the hosts to a third defeat in five home league matches.
Leicester manager Nigel Pearson:
"I think the biggest thing today was that we looked a side having a bit of a crisis of confidence at the minute.
"I can't fault the players for their efforts, but we were not fluid and didn't solve the problems Wednesday gave us.
"They came with a solid foundation and things didn't quite go our way for a number of reasons. Our creativity is not quite there but I don't think there is a fear factor there."
Wednesday manager Dave Jones:
"It shows you how far we have come as a football club because when we played them in November they were far the better team and we had some catching up to do.
"I think we were a bit naive earlier in the season in what we were doing and it took us a little time to adapt. We have lost that naivety and become a bit more solid."
Jose Semedo concedes a free kick for a foul on Sean St. Ledger. Free kick crossed right-footed by Danny Drinkwater from right channel, Anthony Gardner makes a clearance. David Nugent produces a cross, Paul Gallagher takes a shot. Save by Chris Kirkland.
Foul by Jose Semedo on Wes Morgan, free kick awarded. Free kick crossed by Danny Drinkwater, save by Chris Kirkland.
Substitution Ritchie De Laet goes off and Sean St. Ledger comes on.
The ball is swung over by David Nugent.
Inswinging corner taken by Anthony Knockaert from the right by-line, Headed effort from inside the area by Michael Keane hits the post. Inswinging corner taken right-footed by Paul Gallagher.
Substitution Jose Semedo replaces Leroy Lita.
Corner from the right by-line taken by Anthony Knockaert, save made by Chris Kirkland.
Substitution Ben Marshall goes off and Paul Gallagher comes on.
Unfair challenge on Danny Drinkwater by Lewis Buxton results in a free kick. Free kick taken by Danny Drinkwater.
Assist by Lewis Buxton.
69:01 Goal scored
Goal! - Connor Wickham - Leicester 0 - 1 Sheff Wed Connor Wickham scores a goal from close in high into the middle of the goal. Leicester 0-1 Sheff Wed.
Foul by Lewis Buxton on Anthony Knockaert, free kick awarded. Ben Marshall delivers the ball from the free kick right-footed from left channel, save by Chris Kirkland.
Connor Wickham concedes a free kick for a foul on Anthony Knockaert. Ritchie De Laet takes the free kick.
Effort from deep inside the area by Ben Marshall clears the bar.
Unfair challenge on Andy King by Jeremy Helan results in a free kick. Direct free kick taken by Ben Marshall.
Substitution Anthony Knockaert on for Harry Kane.
Unfair challenge on Michail Antonio by Jeff Schlupp results in a free kick. Danny Pugh crosses the ball in from the free kick.
Ritchie De Laet is caught offside. Free kick taken by Chris Kirkland.
Substitution Connor Wickham is brought on as a substitute for Reda Johnson.
Danny Drinkwater takes the free kick.
Booking Lewis Buxton goes into the referee's book for unsporting behaviour.
Free kick awarded for an unfair challenge on Ritchie De Laet by Lewis Buxton.
Michail Antonio takes a shot. Kasper Schmeichel makes a comfortable save.
Inswinging corner taken from the right by-line by Danny Pugh, free kick awarded for a foul by Miguel Llera on Kasper Schmeichel. Free kick taken by Kasper Schmeichel.
A cross is delivered by Ben Marshall, Close range headed effort by Harry Kane clears the crossbar.
Ritchie De Laet fouled by Reda Johnson, the ref awards a free kick. The free kick is swung in right-footed by Ben Marshall.
The official flags David Nugent offside. Chris Kirkland takes the free kick.
The second half has started.
45:00 +1:13 Half time
Half Time The half-time whistle blows.
Inswinging corner taken by Ben Marshall from the left by-line, save by Chris Kirkland.
Shot by Ritchie De Laet. Save made by Chris Kirkland.
Corner from the right by-line taken by Ben Marshall, save by Chris Kirkland.
David Nugent gives away a free kick for an unfair challenge on Lewis Buxton. Lewis Buxton takes the free kick.
Michail Antonio crosses the ball, Leroy Lita takes a shot. Save made by Kasper Schmeichel.
Ben Marshall takes a shot. Save by Chris Kirkland.
Inswinging corner taken by Michail Antonio from the left by-line, Anthony Gardner gives away a free kick for an unfair challenge on Wes Morgan. Kasper Schmeichel takes the free kick.
Ben Marshall takes the inswinging corner, save by Chris Kirkland.
Chris Wood challenges Miguel Llera unfairly and gives away a free kick. Chris Kirkland takes the free kick.
Shot from outside the area by David Nugent misses to the left of the target.
David Nugent is ruled offside. Chris Kirkland takes the indirect free kick.
Wes Morgan challenges Leroy Lita unfairly and gives away a free kick. Free kick crossed by Lewis Buxton, save by Kasper Schmeichel.
Direct free kick taken by Lewis Buxton.
Booking Booking for David Nugent for unsporting behaviour.
David Nugent concedes a free kick for a foul on Lewis Buxton.
Michail Antonio gives away a free kick for an unfair challenge on Wes Morgan. Kasper Schmeichel restarts play with the free kick.
Corner taken by Michail Antonio from the left by-line, Wes Morgan makes a clearance.
The ball is delivered by Michail Antonio, Leroy Lita has a headed effort at goal from close range which goes wide left of the target.
Shot from outside the box by David Nugent misses to the left of the target.
Andy King challenges Michail Antonio unfairly and gives away a free kick. Lewis Buxton takes the free kick.
Inswinging corner taken by Ben Marshall, Michael Keane has a header from close range and clears the crossbar.
Inswinging corner taken by Ben Marshall, Reda Johnson manages to make a clearance.
David Nugent takes a shot. Save made by Chris Kirkland.
Free kick awarded for an unfair challenge on Leroy Lita by Wes Morgan. Michail Antonio takes the free kick.
David Nugent fouled by Reda Johnson, the ref awards a free kick. Ben Marshall crosses the ball in from the free kick, Kieran Lee manages to make a clearance.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.