The common brimstone is a less deep yellow than its southern European close relative the Cleopatra, the males of which have orange patches on their forewings. In other respects these two species are very similar, with wing shapes which when closed look remarkably like pale green leaves. The raised veins on the undersides of the wings of this and other Gonepteryx species when at rest further reinforce the illusion of leaves. (And the wings are invariably closed when this butterfly is at rest.)

These large and impressive butterflies have a wingspan ranging from 6 to 7.4cm. The males are on average slightly smaller than the females.