Barnsley made it three wins from four games under Valérien Ismael with first victory in Derby since 2009.
Goals from Conor Chaplin and Victor Adeboyejo sealed a comfortable road-victory for the in-form Reds on Saturday afternoon.
The Reds made a bright start to proceedings with the returning Cauley Woodrow in the thick of the action early on. The forward linked brilliantly with Callum Styles on the left before his little touch round the corner saw Dominik Frieser in on goal but from eight yards out the Austrian saw his powerful shot turned behind by Derby 'keeper David Marshall.
From the resulting corner, Woodrow was unable to keep his header down as he'd pulled away into space at the far post.
The hosts immediately sprung into life and a nothing ball over the top saw Martyn Waghorn in behind Mads Andersen but the forward couldn't beat Jack Walton.
Woodrow again glanced a header over the crossbar after Callum Brittain had done superbly to beat his man on the right before putting in an inviting right-foot delivery.
Walton had to be at his sharpest to deny Tom Lawrence twice in quick succession, firmly parrying away two efforts from inside the box.
Just after the half-hour mark, Barnsley took the lead, and in strange circumstances.
Marshall on the edge of his own penalty area aimed a low pass to a defender on his right, but he got it wrong as Conor Chaplin pounced before firing home his second goal of the campaign from just inside the box for 1-0.
Some more neat football in and around the Rams' area saw Woodrow slip Alex Mowatt in, 16 yards from goal, but the skipper dragged his left-footed effort a yard wide of goal with Marshall beaten.
Chaplin saw a late free-kick drop just wide of the post, but it had been a pretty entertaining opening 45, and one the Reds probably edged.
HALF-TIME: Derby 0-1 Barnsley
We pretty much opened up the second half how we'd ended the first, with a very close effort from Chaplin, via a set-play - it whistled over the bar.
The Reds were dominant in the early moments once more but Derby were putting bodies on the line to prevent a second Barnsley goal.
A couple of bookings for Mads Andersen and Michael Sollbauer on the hour had us concerned, with the home side known for their ability from set-pieces. But Waghorn's sights weren't in, floating one just wide from 25-yards out.
There were introductions from the bench for Herbie Kane (replacing Matty James) and Victor Adeboyejo (Dominik Frieser), and the latter would soon make a real impact.
All of Chaplin, Kane and Mowatt could have doubled the Reds' lead in the next 10 minutes but were either off-target or thwarted by Marshall in goal.
But Victor wasn't to be stopped. The subsititute picked up the ball 60 yards from goal, he drove at the defence before curling home a superb effort from 20 yards to seal a massive three points for Barnsley.
FULL-TIME: Derby 0-2 Barnsley
Walton, Sollbauer, Helik, Andersen, Brittain, Mowatt, James (Kane), Styles, Frieser (Adeboyejo), Woodrow (Halme), Chaplin.
Unused: Collins, Oduor, Thomas, Schmidt.