A second-half winner from Iain Hume handed Leicester three priceless points in their relegation battle.
Barnsley had the better chances in a tight encounter, but Diego Leon was off target and Martin Devaney hit the post in the first-half.
Daniel Nardiello was denied by Paul Henderson while Barry Hayles went close for the Foxes.
Hayles set up the winner when he squared for the on-rushing Hume to tap in for his 10th of the season.
Barnsley manager Simon Davey:
"We did not deserve to lose. We have been hit by a real sucker punch. To hit the post, create chances and still not win is hard for us to take and it hurts.
"We need to finish the job of escaping relegation now. We face Charlton next week and although it appears that they have nothing to play for, that could work against us.
"I still believe, and always have, that we can stay up. We have got to go out and do it now. The next two games are vital and we must finish strong."
Leicester boss Ian Holloway:
"We came here with everyone saying that we were doomed and we came away with the win.
"I have never known a season like this. You just can't predict what is going to happen. I really don't know who is going to stay up. I think there are a lot more twists and turns to come.
"Nothing has been decided today and we know that. It will be tough but if we win next weekend we will be safe and that has to be the goal for us."
Barnsley: Steele, Foster, Nyatanga, Souza, Kozluk, Leon, Hassell (Coulson 85), Howard, Devaney (Campbell-Ryce 80), Macken, Nardiello (Odejayi 68).
Subs Not Used: Letheren, Ferenczi.
Booked: Kozluk.
Leicester: Henderson, Stearman, McAuley, Kisnorbo, Clapham, Bell (Campbell 90), Wesolowski, Oakley, Hendrie (Hayles 43), Howard, Hume (N'Gotty 84).
Subs Not Used: Alnwick, Laczko.
Booked: Hayles.
Goals: Hume 72.
Att: 14,644.
Ref: Mark Halsey (Lancashire).
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