The pacy striker headed his fifth goal in as many games to set Reading on their way to victory, Andy Hughes adding the gloss in the final minute.
Forster caused no end of problems for a stretched Millwall defence and was unlucky notto claim a spectacular first-half hat-trick.
He put Reading in front inside six minutes with a clinical header from a tight angle.
James Harper's searching cross was missed by Lions keeper Tony Warner, who wasapparently unaided by the Football League's new day-glow ball and Forster met his headersweetly to continue his scoring run.
Warner almost teed up Forster for a remarkable header four minutes before half timewhen he hacked clear under pressure from Harper and could only watch as Forster'sspeculative effort on the turn flew agonisingly wide from 40 yards.
But the Millwall keeper redeemed himself with a superb reaction save to push. Forster'sflicked header on to the crossbar on the stroke of half time.
After those first-half reprieves, Millwall pushed hard for a way back after the break andprompted some frantic defending from the Reading backline.
But the closest Millwall came to a leveller was a Dennis Wise's shot which Adie Williams didbrilliantly to block as the visitors failed to force a single save from Reading keeper MarcusHahnemann.
And a second successive clean sheet was complemented by Reading's crucial second goaltwo minutes into injury time.
Darren Ward felled Martin Butler on the edge of the box and John Salako's rolled free-kickset Hughes up for a drive into the corner of Warner's net.
Victory was especially sweet for the Royals as it came at the expense of their formermanager Mark McGhee