Rosettea princeps is a soft, herbaceous shrub, 1.0 to 3 m tall. Young stems are angular and the whole plant is covered in short, bristly hairs. The leaves are velvety, large, 30-145 x 10-55 mm, egg-shaped to lance-shaped, dark green above and paler to whitish underneath with 5 conspicuous veins from the base. Old leaves turn red. The flowers are large, ± 60 mm diameter, purple, lilac or occasionally white, produced in showy terminal panicles. At Kirstenbosch they are in flower from mid to late summer until autumn, whereafter they are cut back and allowed to go dormant during winter. Further north in more tropical climates, they continue flowering through winter into spring. The calyx is a ± 10 mm long tube with 5 lobes, dark reddish pink, covered in clusters of bristly hairs and knobby bumps, the calyx persists after the petals have dropped off but the calyx lobes fall off. The spectacular inflorescence is multi-toned; the dark reddish pink calyx encloses the dark purple bud that opens to the paler purple to lilac, 5-petalled flowers with their purple and yellow stamens and purplish pink style. The fruit is a capsule that develops inside the persistent calyx, releasing masses of very small seeds.

Rosettea princeps has unusual stamens: firstly, when in bud, they are bent downwards and the anthers are partly hidden inside pockets between the calyx and ovary; secondly, when mature, the anthers shed their pollen through an apical pore; and thirdly, and more easily observed by a person without a dissecting microscope, the stamens are unequal. Looking at a flower, it appears as if the two types of stamens are easy to spot and very clearly different - there is a bunch with what appear to be yellow anthers curving downwards on the top half of the flower, and a bunch with purple anthers curving upwards on the lower half. In fact, the yellow bits are not anthers at all, but an appendage on the filament. It also looks as if there are many stamens, butthere are only 10 stamens, attached just inside the rim of the calyx, in two rows: 5 alternate with the petals while the other 5 are attached opposite the petals. Why it looks as if there are more than 10 is because each stamen is made up of two parts: the ± 10 mm long yellow filament and the dark purple anther that is attached to the filament by a pinkish purple, jointed connective. At the point where the connective joins the filament there are two short, yellow horns-these are what appear to be yellow anthers. The actual anthers are dark purple and ± 10 mm long and crinkled when mature. All the anthers are fertile.The stamens of the opposite row have 5 mm long connectives-you can see them clustered in the middle of the flowers directly under the yellow filaments and horns. The stamens of the alternate row have longer 15 mm connectives-they are the large claw-like structures in the lower half of the flower. The style is ± 30 mm long, purplish pink with a yellow stigma.

Two varieties are recognised, the typical variety and var. candolleana which is distinguished by the bristles on the calyx tube being less dense, about 1.5 mm long, and not obscuring the calyx tube whereas in the variety princeps the bristles on the calyx tube are relatively dense and more or less obscure the calyx, especially the lower half when young and the bristles are about 2.5 mm long. The inflorescence is usually lax in var. candolleana and condensed in var. princeps.