The short-lived, nocturnal flowers of Piaranthus atrosanguineus grow in fascicles of up to three from the central parts of young stems, opening successively. The flowers exude a strong, foetid odour, sometimes described as sour and musty. Flowers open late afternoon and wilt the next morning, the shortest bloomtime of all stapeliads, requiring little moths or perhaps some flies to be quick and diligent.
The flower pedicels are stout and hairless. The small sepals are lance-shaped with acute, spreading tips.
The five-pointed, star-shaped corolla of the flower is grey-green, mottled purple on its outside, while spectacularly dark crimson or maroon on the inside. The convex corolla lobe surfaces are slightly wrinkled and covered in small papillae; sometimes also hairy towards the lobe tips that taper to acute points, each ending in a tiny protuberance. The lobe margins are slightly rolled under. Flower (corolla) diameter is 3 cm to 4,5 cm.
The outer corona at the base of the flower centre is inconspicuous, its dull orange-yellow lobes spreading horizontally, each with a central, longitudinal channel in which the nectar is secreted.
The tall inner corona has five conspicuous linear and terete pale lobes, up to 9 mm long, white-tipped and incumbent upon the anthers. These wiry lobes converge in the centre, diverge at the top.
The blooming season of this plant is late spring to autumn and rain-dependent (Hardy and Fabian, 1992; White and Sloane, 1937; www.redlist.sanbi.org).