Goals from Iain Hume and Stephen Foster secured victory for Barnsley and lifted them off the foot of the Championship.
After only a few half-chances in the opening 45 minutes, Rob Hulse's long-range shot forced a good save from Barnsley goalkeeper Luke Steele.
But Barnsley took the lead on 72 minutes when Hume crashed in a spectacular volley.
And the points were sealed when Hume crossed for Foster to head in a second goal seven minutes from the end.
Barnsley boss Simon Davey:
"I'm really pleased for the players because they worked really hard in the first four games but got nothing.
"It was a scrappy game but we took our chances when they came to us and got the rub of the green which we didn't get in the previous four games. We've got three points and we're going to move on from here."
Derby boss Paul Jewell:
"That's about as well as we've played this season. I'm very disappointed that we've lost the game but I'm pleased with the performance.
"We were by the far the better team but we probably just didn't have the cutting edge and they scored a goal out of nothing. I thought there were some good points so there is some light at the end of the tunnel."
Barnsley: Steele, Foster, Moore, Guedes, Kozluk, El Haimour (Odejayi 63), Leon (Hassell 67), Howard, Devaney, Hume, Macken (Mostto 90).
Subs Not Used: Muller, Van Homoet.
Booked: Foster, Howard.
Goals: Hume 72, Foster 83.
Derby: Carroll, Connolly, Leacock, Albrechtsen, Stewart, Green, Kazmierczak, Addison, Commons (Davies 84), Villa (Ellington 77), Hulse (Barazite 83).
Subs Not Used: Bywater, Davis.
Booked: Addison.
Att: 14,223
Ref: Graham Salisbury (Lancashire).
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