Paul Peschisolido marked his debut with a second half winner that boosted Derby's survival hopes and dragged Rotherham back into the relegation scrap.
The Canadian striker netted at the second attempt to settle a game that Derby desperately needed to win.
It was his third goal against Rotherham this season and manager Ronnie Moore admitted: "He must love playing against our back four. How he was left alone like that was ridiculous." For Derby manager George Burley, Peschisolido provided the presence in the penalty box Derby have been missing most of the season.
He said: "Paul was outstanding, not only for his goal, but for his overall work rate and commitment.
"There has been quite a few games this season when we've been on top, but we've not had a natural scorer and that's why I brought Paul in." For much of the first half, it threatened to be a frustrating afternoon for Peschisolido who had a couple of half chances, but little else as Rotherham denied Derby the space to put any moves of quality together.
The best chance of the first 45 minutes fell to the visitors, but Paul Warne blazed a shot wastefully over from just outside the area when he broke through the middle in the 33rd minute.
Rotherham did not have another chance as good as that again and Derby turned the match with a second-half reshuffle.
The introduction of winger Lee Holmes on the left allowed Ian Taylor to move into the centre of midfield and the home side started to pose problems.
The winning goal was well worked with Marcus Tudgay releasing right-back Richard Jackson with a clever flick.
His low cross from the right was volleyed by Peschisolido and although Mike Pollitt made a splendid save, the striker pounced on the rebound to score from six yards.
Rotherham could not find a reply although Darren Garner should have done better with a header from 12 yards out 20 minutes from the end.
The Millers will hope that Jody Morris, who has signed until the end of the season, will give them the extra quality that they badly lacked in this match.