Geniostoma lydgatei is a shrub or small tree. The stems are sharply angled, apparently many-branched, and sparsely short-hispidulous. The leaves are simple, opposite, with blades elliptic to oblanceolate, 5–10 cm long, and 2–7 cm wide. The apex of the leaves is acuminate, and the base is attenuate to cuneate. The upper surfaces of the leaves are glabrous, while the lower surfaces are moderately appressed, short-hispidulous along the veins, and the blades are slightly rugose. The margins are entire or rarely toothed. The petioles are 0–0.8 cm long, and the stipules are interpetiolar, possibly intrapetiolar, membranous, connate into a cup about 4–5 mm long, distinct from the petioles, glabrous or sparsely hispidulous near the base, and deciduous. The flowers are in terminal inflorescences, with 6–20 flowers in compound, sessile cymes, which are short-hispidulous throughout or glabrate. The pedicels are 6–12 mm long, and the bracts and bracteoles are filiform, 1.5–2.5 mm long. The flowers are bisexual (perfect) or functionally unisexual and actinomorphic (radially symmetric). The calyx is 5-merous, with unequal lobes united at the base, linear-subulate, 1.5–2(–2.5) mm long, and 0.5–0.8 mm wide, imbricate, glabrous, with the inner surface often having collectors at the base, and the margin mostly fimbriate. The corolla is 5-merous, probably pale yellow or greenish yellow, funnelform-salverform, 6.5–7 mm long, with the tube about 4 mm long, glabrous externally, and the lobes narrowly deltate, 2.5–3 mm long, sparsely hirtellous on the tube within and on the lower part of the lobes. The lobes are imbricate or contorted in bud. The stamens are 5-merous, as many as and alternate with the corolla lobes, attached to the mouth of the corolla, exserted, usually recurved after anthesis, and glabrous or hairy, with primarily short filaments. The anthers are dithecal, opening by longitudinal slits. The ovary is superior or partly inferior, with carpels hairy or glabrous, 2- or 3-locular, sometimes with imperfect partitions in the upper part of the ovary. The ovules are numerous and amphitropous, with axile placentation. The style is terminal and mostly short, with a clavate stigma, ellipsoid to globular, sometimes ± oblong, about as large as the ovary. The fruit is an ovoid capsule, 3.5–4 mm long and 3–3.2 mm in diameter, 2-valved, with valves not keeled, an apex with a small blunt beak 0.2–0.3 mm long, septicidal to sub-septifragal, maturing to green, sometimes with a black-purple tinge, and usually becoming brown-black when over-mature. It is found in the montane Wahiawa Mountains on Kaua’i between 300–1100 meters.