Albion's rough run of luck continued as a hotly disputed goal sentenced them to their eighth League defeat on the trot.
Referee Mike Pike overruled his linesman's offside flag when Alan Lee struck the killer blow after 20 minutes.
Players from both sides stopped when the linesman flagged with the ball at Lee's feet 30 yards out.
The Republic of Ireland marksman looked yards offside as he nonchalantly lobbed the stranded Andy Petterson.
Few of the players noticed, however, that Pike had indicated play should continue and, after consulting his assistant, the official from Barrow-in-Furness allowed Lee's seventh of the season to stand.
Pike apparently decided that Albion midfielder Richard Carpenter was the last player to touch the ball before Lee as he tackled Darren Byfield.
The Seagulls were guilty of not playing to the whistle, although that is easier said than done in such circumstances.
Rotherham were good value for their lead in the first half, despite the manner in which it was attained.
Albion, with new signing John Piercy, making a satisfactory debut in midfield, battled their way back into the match in the second half.
Gary Hart nearly stunned a crowd of 6,696 at Millmoor with a volley into the ground which was pushed behind by keeper Mike Pollitt.
Captain Danny Cullip was outstanding for Martin Hinshelwood's men in a defence including the recalled Kerry Mayo.
The Seagulls, already depleted by injuries, suffered another blow when Nathan Jones limped off after 55 minutes.
They desperately need the likes of Bobby Zamora, Paul Kitson and Paul Brooker back following a sixth blank sheet out of ten in Division One.
Albion have now taken one point from nine games since the opening day victory at Burnley.
They will be hoping to have some of the injured back before next Saturday's crunch clash against fellow strugglers Grimsby at Withdean.
Before that they face a tough Worthington Cup assignment at Ipswich on Tuesday.
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