Geniostoma hirtellum is a shrub or small tree reaching 1.5 to 6 meters. The stems are usually sharply angled or winged, with old wings often undulating and irregularly many-branched. The young branches are short-hirtellous or, in some cases (such as on Kaua’i), glabrous. The leaves are simple and opposite, with elliptic blades broadly elliptic or oblong-elliptic, occasionally obovate or narrowly elliptic on Kaua’i. They range in size from (4.5–)6 to 19 cm in length and 1.5 to 8.6 cm in width. The apex is short-acuminate, and the base is cuneate. The upper surfaces of the leaves are glabrous, while the lower surfaces are moderately to sparsely short-hispid or sometimes glabrous on Kaua’i. The leaves are green and glossy on the upper surface and pale green on the lower surface, with a membranous to chartaceous texture. Margins are entire or rarely toothed, and petioles are 0.7–2.7 cm long. Stipules are interpetiolar or sometimes intrapetiolar, membranous, and connate into a cup 2–6(–8) mm long, adnate to the petiole in the lower third to nearly throughout, and are deciduous. The flowers are in terminal inflorescences, (3–)6–25(–50) in compound or sometimes simple, open or congested, sessile, paniculate cymes. The inflorescences are densely short-hispidulous to glabrous throughout, with pedicels measuring 5–16 mm long, usually reflexed. They are bisexual (perfect) or functionally unisexual and actinomorphic (radially symmetric). The calyx is 5-merous with sepals united at the base and slightly unequal. The corolla is also 5-merous, campanulate to rotate, and varies in color from greenish-yellow to pale orange or yellow, sometimes cream. The fruit is a capsule, ellipsoid to occasionally obovoid, usually somewhat compressed, measuring 15–34 mm in length and 8–15(–22) mm in diameter. Seeds are ellipsoid to subglobular, black to brown, approximately 1.5 to 2 mm long, and may be winged at both ends or all around. They are embedded in juicy orange to red-pulp. The seed surface is mostly minutely papillose to areolate, and the endosperm is oily with a small embryo. It is scattered in mesic to wet forests between 500-1,830 meters.