Josh Coburn scored a stoppage-time winner to snatch Millwall a 1-0 victory at Pride Park and plunge Derby deeper into relegation trouble.
Coburn had seen a late penalty saved but he was in the right place in the fifth minute of time added on to turn in the rebound and secure a win that keeps the Lions in the play-off race.
Derby remain rooted in the bottom three and have now failed to win or score a goal at home this year.
Millwall looked the more threatening in the first 15 minutes and had penalty claims waved away after Japhet Tanganga went down under Matt Clarke’s challenge before Joe Bryan fired just wide from 20 yards.
The Rams finally fashioned a decent chance in the 36th minute with Kenzo Goudmijn getting in behind on the left but his cross was headed wide by Ebou Adams.
Derby had to improve in the final third in the second half but a heavy touch from Lars-Jorgen Salvesen saw a decent move break down in the opening minutes.
Salvesen did better shortly afterwards but his low ball in was cleared before Jerry Yates could get in a shot.
Millwall lost the impressive Bryan in the 58th minute after he was caught by Ryan Nyambe who was booked, and Derby almost took the lead five minutes later.
A corner was headed powerfully by Clarke against Femi Azeez and onto the underside of the crossbar but when the ball came down it fell into the hands of goalkeeper Lukas Jensen.
It was the closest there had been to a goal in a game short on quality, but Millwall had a great chance to win it in the 84th minute.
Coburn went down under the challenge of Sondre Langas as he ran across the box and referee Elliot Bell pointed to the spot.
It looked harsh but Coburn could not take advantage as his penalty was saved by Jacob Widell Zetterstrom diving to his right.
Both teams had chances to win it in eight minutes of added time and it was Coburn who made amends at the back post to tap in the rebound after Zetterstrom had saved from George Saville.
