Brynjar Gunnarsson scored twice to sink Sheffield United and halve their lead at the top of the Championship to three points.
The match appeared to be heading for a draw when Gunnarsson - who had put second-placed Reading ahead in the first half only for Steve Kabba to equalise - headed past Paddy Kenny a minute from time.
The result was harsh on Sheffield United, who played their part in a fine match and deserved a draw.
But Reading started and finished the stronger, and that proved decisive. They set a furious pace from the start, and it paid dividends as they took an instant lead.
Glen Little had already gone close with a shot which was deflected wide, and from the resulting corner he picked out the unmarked Gunnarsson, who volleyed past Paddy Kenny from 15 yards.
But United had won 10 of their first 11 games this term, and it was inevitable the Yorkshiremen would force their way back into the match.
Neil Shipperley and Rob Kozluk had already wasted good chances when, after 15 minutes, Paul Ifill did superbly to cross from the right.
Alan Quinn was waiting at the far post and turned the ball back into the centre, where Kabba flung himself forward to head past Marcus Hahnemann from six yards.
The visitors then enjoyed a huge slice of luck when Kenny seemed to handle Nick Montgomery's wayward back-header outside his own box, preventing a probable own-goal.
But referee Grant Hegley showed the Irishman only a yellow card, to the fury of the home fans.
Six minutes before half-time, Montgomery surged forward from central midfield before powering a 30-yard drive just over Hahnemann's crossbar.
The pace of the early stages of the second half was far less frantic than the first, and the defences were nothing like as busy.
Indeed, there were only two chances in its first 20 minutes.
First Royals defender Ivar Ingimarsson almost scored a bizarre 61st-minute goal when Kenny's save from Leroy Lita's header rebounded off the Icelander but trickled just wide.
And two minutes later, Stephen Hunt was forced to head clear inches from his own goal-line after Shipperley had nodded David Unsworth's right-wing corner goalwards for United.
Kabba's speed and power were a constant menace for the home defence, and in the 70th minute he charged past Ibrahima Sonko before pulling the ball back, only for substitute Sekou Baradji to make a vital interception.
Back came Reading and another substitute, John Oster, hit a fierce 25-yard volley narrowly wide.
United then had reason to feel annoyed when substitute Keith Gillespie seemed to be body-checked by Ingimarsson on the edge of the area, but Hegley waved play on.
As the game seemed to be heading for a draw, Gunnarsson rose to head Baradji's free-kick into Kenny's bottom-right corner.