John Salako came back to haunt Steve Coppell as Albion were condemned to a second defeat in four days.
The former England winger's 22nd minute strike earned Reading a remarkable sixth win in succession without conceding a goal.
Salako, who played under Seagulls' chief Coppell at Crystal Palace, cracked a fierce left-foot shot through Michel Kuipers from Nicky Forster's square pass.
Albion, beaten 3-2 on Wednesday at another of the promotion contenders Nottingham Forest, rarely threatened Reading's proud defensive run, despite the return from injury of Bobby Zamora.
Coppell tried everything he knew at a rain-soaked Withdean in a bid to break them down.
He changed from 5-3-2 to 4-4-2 in the first half, then brought on Paul Brooker for Nathan Jones after 53 minutes.
Brooker could have equalised seven minutes later after a mistake by Reading Captain Adrian Williams, but his shot was saved by Marcus Hahnemann.
Coppell threw on young striker Daniel Webb in place of Richard Carpenter with 14 minutes left.
Albion forced a flurry of corners in the closing stages and from one of them Kerry Mayo's header forced Hahnemann into another good stop.
The Seagulls seventh defeat out of ten at home in front of a crowd of 6,817 means they are still rooted to the foot of the table.
Elsewhere at the bottom Sheffield Wednesday were beaten 3-0 at Wimbledon and Bradford 1-0 at Millwall.
Grimsby lost 2-1 at home to Micky Adams' ten-man Leicester, but Stoke ended their losing sequence with a 1-1 draw at Gillingham.
Graham Barrett, missing again today with a knee injury, is expected to stretch his loan move from Arsenal to the rest of the season before next Saturday's relegation showdown at Sheffield Wednesday.
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