Geniostoma waiolani is a branched shrub reaching heights of 1–3 meters. Its stems are angled or slightly winged, covered with a moderate to dense layer of short hispid hairs. The leaves are simple and opposite, with narrowly elliptic to elliptic or elliptic–oblanceolate blades measuring 4–12 (–16.5) cm in length and 1.5–4.5(–7.3) cm in width, featuring an acute to abruptly acuminate apex and a cuneate or attenuate base. The leaves have a glabrous upper surface and a sparsely hispidulous lower surface, especially when young, with a pale green, glossy, and membranous texture. The flowers occur in terminal inflorescences with one or sometimes up to 11 flowers in sessile cymes, with pedicels ranging from 3–7 mm long. The calyx has five large, foliaceous lobes that are imbricate and distinctly unequal. The corolla is dark to golden yellow, 15–25 mm long, and campanulate to rotate with narrowly deltate lobes. Stamens are exserted and recurved post-anthesis. The fruits are broadly ellipsoid to ovoid–ellipsoid capsules, 11–17(–24) mm long, maturing to green and usually turning brown-black when over–mature, containing numerous, reddish-brown seeds that are embedded in juicy orange to red pulp. These seeds are ellipsoid to subglobular; intruded on the hilar side; surface mostly minutely papillose to areolate; reddish brown; ca. 1.4–1.5 mm long; endosperm oily; proteinaceous; with saccharose or hemicellulose; embryo small; straight. It is found in mesic to wet forests between 360–1,280 meters.