Derby's three-match unbeaten run in the Sky Bet Championship came to an end as Troy Deeney made a winning debut in Birmingham's 2-0 victory at St Andrew's.
Scott Hogan scored Birmingham's first home league goal of the season - his 50th in the Championship - before Jeremie Bela sealed the points late on.
Striker Deeney, a lifelong Birmingham supporter, came off the bench in the 68th minute following his free transfer from Watford to the delight of the home crowd.
The win moved Lee Bowyer's side up to fourth ahead of Saturday's matches.