A wretched week for Stoke ended with a third straight defeat and the visitors can have no complaints about this result as they caved in at the back after taking an early lead.
A header from Gerry Taggart put Stoke in front but within a minute they allowed Derby to equalise and two more defensive blunders sealed their fate.
For Derby, this was a vital victory as all the teams around them also won, but an injury to key striker Grzegorz Rasiak will give them some anxious days ahead of next week's showdown at Sheffield United.
Rasiak scored his 17th goal of a highly impressive season and Spanish midfielder Inigo Idiakez also took his tally into double figures.
Derby were worthy victors but they got more than a helping hand from Stoke who were well below the form which had given them a chance of the play-offs before a disastrous eight days.
Home defeats by Rotherham and Cardiff put them out of the race but they were given the perfect start here when Taggart got on the end of a free-kick from David Brammer to plant a header inside Lee Camp's left hand post.
But the normally dependable Taggart blundered a minute later when he allowed a low cross to reach Rasiak and the striker cleverly skipped away from two tackles before sliding a shot inside the near post from a tight angle.
Stoke were badly at fault for the second which arrived in first-half stoppage-time. Idiakez pulled a corner back to the unmarked Morten Bisgaard who swept a low shot into the net from 15 yards.
At least Stoke looked a livelier team in the second half and they started to give Derby some anxious moments with Camp forced into a full-length save from Darel Russell in the 74th minute.
But minutes later Idiakez delivered a corner to the near post and the ball was missed by two players before it went in off the keeper.
It summed up Stoke's afternoon, but Derby took advantage and remain on course for a top-six finish.