Thilachium africanum is a deciduous woody shrub or small tree, which grows up to 4 m tall. The main stem is greyish brown; young branch tips are rough and branches are covered with lenticels. The leaves are compound, trifoliolate, occasionally simple, alternate, elliptic to obovate, 30–90 × 10–45 mm, dark green, leathery, hairless, bristle tipped; apex tapering or rounded; margin entire and rolled under. The petiole is rough to the touch (scabrid) and up to 60 mm long.

The flowers are large, without petals, with numerous white stamens, up to 45 mm long, that turn pink as they age. Flowers are borne in small clusters in spring (Aug. –Oct.). As the flower opens, the calyx splits in half horizontally with one half remaining like a cap attached on one side. The fruit is green, purple when ripe, and looks like a cucumber, hence the common name, with 12 longitudinal ridges, up to 60 mm long and 25–30 mm wide, borne in spring and early summer (Oct. –Dec.).