Rotheca hirsuta is a perennial herb with many erect, unbranched, angular, annual stems arising from an underground rootstock. The stems are mostly up to 0.3 m high, rarely up to 0.5 m. The sessile leaves of R. hirsuta are arranged in whorls of three, sometimes four, rarely opposite. The shape of the blade varies from narrowly oblong, narrowly obovate, narrowly elliptic, elliptic to broadly elliptic, with an acute apex and entire margin. The leaf blades are totally glabrous and smooth to densely hairy and often somewhat succulent. The surfaces of the leaf blade are often described in the literature as glandular-punctate.

A study of the leaf surfaces with the scanning electron microscope revealed that there are actually numerous peltate hairs covering the leaf surface.

Flowering has been recorded from as early as July, and extends to March. Flowers are borne on flower stalks (pedunculate) in the axils of each leaf, in the lower half of the stem, below the growth point, in 1-3-flowered inflorescences (cymes). The calyx is fused to about halfway, with five free calyx lobes. The corolla appears as a ball on a short tube, opening up as a bilaterally symmetrical (zygomorphic) unit consisting of a short tube, four lateral lobes (two on each side), and a lower lip. The colour of the corolla is dark blue or purplish with a white, inverted Y-shaped pattern on the lower lip. Four stamens and the style are inserted in the corolla throat, opposite the lower corolla lip, long exserted and curved towards it. The stigma consists of 2 unequal lobes.

The superior ovary is 4-lobed in the upper part. One or two of these lobes develop into mature fruits. The fruit is a drupe, deeply 2-lobed or by abortion 1, elliptic to roundish, up to 15 mm long, black when mature. The fruits are large and heavy in comparison to the small plant carrying them. The calyx at the base of the fruit is persistent and cup-shaped.