Huddersfield lost winger Rajiv van La Parra midway through the first half when he was sent off for a second booking - for dissent - in the space of three minutes.
Reading made it eight games in a row undefeated with a goal four minutes from half-time, when Roy Beerens steered in a deflected shot, and then held on to thwart a late Huddersfield rally.
The Terriers drop from first place to second in the table.
Huddersfield had taken 19 points - from six wins and a draw - from their opening eight matches, the best start to a season in the club’s history.
All of their six victories had been by a single-goal margin, either 2-1 or 1-0.
Reading were unbeaten in seven matches in all competitions and in midweek reached the last 16 of the EFL Cup, in which they will play at Arsenal.
But they faced Huddersfield without the suspended duo of Garath McCleary and Tyler Blackett.
Huddersfield started the stronger, with right-back Tommy Smith getting forward to test home goalkeeper Ali Al Habsi with a low shot.
Reading relied on retaining possession as long as possible but Beerens should have done better when finding space in the Huddersfield area.
However, with keeper Danny Ward exposed, Beerens lost control at the vital moment and the chance was lost.
Huddersfield responded with a neat move involving Jonathan Hogg and Kasey Palmer.
The ball was worked wide to Elias Kachunga but the winger scuffed his shot across the face of the goal.
A scrappy affair sprung to life in the 26th minute when Van La Parra was sent off for a second bookable offence.
Cautioned for a foul only three minutes earlier, Van La Parra won a free-kick but continued to harangue referee James Linington and was dismissed for dissent.
Reading almost made Huddersfield pay immediately, when John Swift thundered a 30-yard drive onto the base of a post as Ward watched helplessly.
But they did go in front in the 41st minute, when Beerens’ wickedly deflected shot off Smith, from a Callum Harriott cross, deceived Ward.
Huddersfield could have levelled before the break when captain Mark Hudson nodded narrowly over from an Aaron Mooy corner.
Reading continued to make their numerical advantage count in the second half, with Harriott seeing a firm effort blocked.
Beerens then burst through and went past Ward but the angle was too tight and he could find only the side netting.
Huddersfield kept battling, frequently probing forward, but they had no cutting edge and failed to find an equaliser despite a brave late flurry.
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