Theo Robinson fired Derby to a third straight victory as the Rams' charge towards the play-off places continued at Hull's expense.
Robinson curled home the game's only goal from just inside the penalty area 32 seconds into the second half.
Derby goalkeeper Frank Fielding kept out shots from Jack Hobbs and Matty Fryatt in the first half.
And as Hull threw everything at Derby in the last 20 minutes, Fielding was on hand to save Fryatt's header.
Despite the late onslaught, Hull succumbed to their third defeat in a row over the Christmas period, leaving them clinging on to sixth place, just one point clear of Blackpool.
Hull had dominated the first half and were stunned when the visitors took the lead moments after the restart.
James Bailey crossed from the left and, after dancing clear on the left-hand side of the penalty area, Robinson curled a left-footed shot beyond Adriano Basso for his seventh goal of the season.
Buoyed by the breakthrough, the Rams enjoyed their best spell of the game, with striker Jamie Ward and midfielder Paul Green going close.
But Hull responded with a spell of unrelenting pressure as Andy Dawson headed Cameron Stewart's cross over the bar and Robert Koren lashed a long-range shot straight at Fielding.
The away side's keeper was on hand to deny Fryatt and Koren in the last 10 minutes as Derby held on to move into ninth place.
Hull caretaker manager Nick Barmby: "At half-time we told the players that Derby had come to frustrate us, but we were done by a sucker punch.
"They scored the goal and then parked the bus, but all credit to them as they did it well.
"Our passing wasn't fluid and we didn't create too many chances. Our reputation's out there as a good passing side, but we've got to make things happen."
Derby first-team coach John Metgod: "The senior players and the younger players have done an amazing job and the team is gelling.
"It might not have been the best performance football-wise. On the ball we were very poor.
"But we changed our system to 4-3-3, got a goal and then we limited them to shots from the edge of the box and they didn't create a real chance."