Steve Coppell's side roared to victory with five goals in 24 second-half minutes after previously struggling to break the Rams down.
James Harper's goal on 59 minutes proved the breakthrough as he poked home after Dave Kitson had played a one-two with the midfielder.
Kevin Doyle headed the second from a Bobby Convey cross and then Coppell sent on the substitutes to finish Derby off.
John Oster scored within seconds of coming on as he swept home Stephen Hunt's cross and Oster then fired against a post after fellow substitute Shane Long had set him up.
Long headed home a cross from captain Graeme Murty and the Irish striker set the seal on victory when he finished off after a superb pass from Hunt.
The final whistle – and subsequent news of Sheffield United's failure to beat Stoke – prompted a mass pitch invasion by the home fans, who celebrated as the players squirted champagne from the directors' box.
But that party appeared a long way off in the first half as Reading looked jaded.
They still enjoyed the better chances though with Steve Sidwell blazing over the bar after a neat one-two with Kitson.
Kitson also got in the way of a goalbound Convey volley as Reading pushed for the win that could have secured the title.
But Derby had their chances too only for Tommy Smith's first touch to let him down after Paul Peschisolido had dispossessed Ivar Ingimarsson.
Smith missed an even better chance early in the second half when he blazed over with just Marcus Hahnemann to beat.
But Reading took control just before the hour mark and Coppell's side never looked back from there.