Hull's npower Championship play-off push maintained its momentum with a first ever victory at Pride Park as Derby's alarming slump continued with a 1-0 defeat.
Defender Anthony Gerrard scored the only goal of the game midway through the second half from Robert Koren's inswinging corner.
Nigel Pearson's side are now nine league games unbeaten and up to eighth in the table on the strength of only one defeat in their last 18 Championship matches.
Miles Addison missed the target with a header early on before former Hull loanee Daniel Ayala did likewise as Derby threatened again in the 19th minute.
Derby were forced to replace goalkeeper Stephen Bywater at the start of the second half, with James Severn coming on for his first league appearance for the club.
Hull were obviously keen to test the youngster early on and, after he made saves from Matty Fryatt and Aaron McLean, he was beaten when Gerrard netted for the third time this season from close range following Koren's curling delivery.