Noel Hunt grabbed the only goal of a dour match at Pride Park to propel Reading into the play-off spots.
The Irishman's header from Jimmy Kebe's cross was his first goal since scoring against Ipswich on 26 November.
Derby's Theo Robinson and Paul Green tried their luck from long range but neither came close to dragging the hosts back to parity.
The three points carry the Royals up to sixth, while Derby end a run of three wins at home.
After a dire first half, Hunt fired two sighters from just outside the box after the break.
The striker found the net just after the hour as referee Geoff Eltringham waved away Derby forward Jamie Ward's appeals for a foul on the edge of the visitors' box, and a swift counter ended with Hunt powering a header past Frank Fielding.
Derby never looked likely to summon up an equaliser and toiled hard without creating any real chances to level.
Derby manager Nigel Clough:
"We didn't get going all night which has puzzled us somewhat because we played well on Saturday and have been good in training in the last two days.
"We absolutely fell flat tonight. We can't put our finger on it at the moment.
"We were playing against a very good side tonight but we didn't cause them enough problems and that's been as poor as we've been for some time."
Reading boss Brian McDermott:
"It was efficient and we needed a little bit of Jimmy Kebe to make the goal for us.
"Derby worked really hard on him tonight but we felt he has always got that bit of quality to make the difference and he's produced a good cross and Noel's got in a good header.
"Eight wins from 10 speaks for itself. We normally leave it until end of March or April so we are happy enough, this is the best position we have been in in the last couple of years."