Newcastle's run of 15 matches without defeat ended unexpectedly and emphatically as Derby, who had beaten then second-placed Nottingham Forest in their last home game, this time toppled the Championship leaders in a performance that made nonsense of their relegation worries.
In fact, the heaviest of Newcastle's four defeats this season cost Chris Hughton's side top place as goals from Rob Hulse, Kris Commons and Shaun Barker enabled Derby to celebrate. The visitors' unbeaten sequence was the club's best in 60 years but West Bromwich Albion go ahead on goal difference following their 2-0 home victory over Scunthorpe.
Derby began only four points outside the bottom three but there had been signs of improvement. The defeat of Forest was followed by a draw at Sheffield United in which only a last-gasp equaliser denied them maximum points. A hint of stability, long sought by manager Nigel Clough, came in the shape of a first unchanged selection in more than a year.
"We've said that we can play anybody if we have our full side out or close to it and anyone who has seen us recently would have said that this result was probably coming," Clough said.
Newcastle had not lost a league match since Scunthorpe beat them in October and the shock was deeper for having thrashed Cardiff 5-1 last time out. But while it took Derby until the 40th minute to get their noses in front, when Hulse headed home a fine Commons cross, they were good value for their lead.
Newcastle forced goalkeeper Stephen Bywater to be at his best to keep out Wayne Routledge and Kevin Nolan before the break. At the start of the second half Andy Carroll headed home Routledge's cross within 20 seconds, only for referee Anthony Taylor to bring play back for a Newcastle free kick. Nicky Hunt's brave tackle then denied Nolan.
An attainable target became much more difficult, however, as Derby scored twice in the space of five minutes, first through Commons from the penalty spot after Michael Tonge had been tripped by Jonas Gutierrez, then Barker's volley after Hulse had looped the ball back into the penalty area following a half-cleared corner.
Hughton's response was to make a triple substitution but Derby were in no mood to relinquish control. "It is a blow but we have been the best team for most of the season and I'm quite sure there will be a desire to put things right in the next game," Hughton said.
Derby County (4-4-2): Bywater; Hunt, Barker, Buxton, McEveley; Tonge, Savage (Anderson, 88), Green, Pearson; Hulse (Martin, 88), Commons (Vidal, 81). Substitutes not used: Deeney (gk), Moxey, Teale, Porter.
Newcastle United (4-4-2): Harper; Simpson, Williamson, Hall, Van Aanholt (Smith, 67); Routledge, Guthrie, Nolan, Gutierrez; Best (Lovenkrands, 67), Carroll (Ameobi, 67). Substitutes not used: Krul (gk), Kadar, Pancrate, Donaldson.
Referee: A Taylor (Greater Manchester).