Trematolobelia singularis grows 0.6–1.5 m tall. The stem is 0.8–1.4 cm in diameter and glabrous, with shallowly obtriangular cicatrices, measuring 1–2 mm tall and 3.5–5 mm wide. The leaves are simple and alternate, forming a dense spherical, oblate, or cylindrical apical rosette. The blades are narrowly elliptic, 12–16 cm long, 1.1–1.5 cm wide, with a length-to-width ratio of 10–13. The apex is acuminate, and the base attenuates, sometimes appearing petiole-like, while in other cases, the differentiation of a petiole is indefinite, making the leaf sessile. The surfaces are glabrous, and the blades are coriaceous with entire margins. Stipules are absent. The flowers are in a terminal raceme, single, erect, with 13–65 flowers, and are glabrous. The peduncle is 15–27 cm long. Sterile bracts are lanceolate, narrowly elliptic, narrowly oblong, or linear, 17–120 mm long, 4–15 mm wide, with entire margins and an acuminate apex. The rachis is 10–29 cm long. Floral bracts are lanceolate or linear, 11–26 mm long, 2.8–4 mm wide, with entire margins and an acuminate apex. Pedicels are 22–30 mm long, and bracteoles are ovate or broadly elliptic, 1.5–3 mm long. The hypanthium is ovoid or oblate, 5–7 mm long, 5.5–7 mm in diameter, 1/14–1/10 as long as the corolla. These flowers are tetracyclic, bisexual (perfect), protandrous, zygomorphic (bilaterally symmetric), epigynous, pedicellate, resupinate, ornithophilous, and large. The calyx lobes are triangular, ovate, lanceolate, or oblong, erect, 4–6 mm long, 2–3 mm wide, and 3/5 as long as the hypanthium, with entire margins and an obtuse apex. The corolla is sub-bilabiate, bright crimson, 45–52 mm long, with a tube that is gently curved, 27–33 mm long, 6–8 mm tall at the middle, 3–7 mm tall at the mouth, and 3.8–5.5 times longer than the height at the middle. The dorsal lobes are 18–24 mm long, 1.5–2.5 mm wide, half to three-quarters as long as the tube, with an acuminate apex. The ventral lip is obovate, 14–18 mm long, 6–10 mm wide. There are five stamens, antisepalous, connate distally for most of their length. The staminal column is scarcely exserted, emerging above or between the dorsal lobes. The filament tube is 37–42 mm long, 3.1–3.8 times longer than the dorsal anthers, pubescent toward the apex. The anther tube is 3.2–4 mm in diameter, pubescent toward the base. The dorsal anthers are 10–12 mm long, and the ventral anthers are 8–9 mm long, with tufts of white trichomes 1.5–2 mm long at the apex. The pollen grains are 41–44 μm polar diameter, 27–30 μm equatorial diameter, prolate, and tricolporate, with a shallowly and minutely reticulate exine. The ovary is inferior, 2-loculed, adnate to the hypanthium, flat at the apex, with axile placentae and numerous ovules. The style is slender and cylindrical, with a ring of stiff white hairs near the apex. The stigma is 2-lobed, with the lobes appressed and non-receptive as the style grows through the anther tube, pushing out pollen, after which the stigmas spread and become receptive. The fruits are oblate capsules, 8–9 mm long, 11–13 mm wide, 8–9 mm thick, with numerous pores scattered over the entire lateral surface. It is endemic to the upper slopes of Kōnāhuanui in the southern Ko‘olau Mountains of O‘ahu, growing in lowland wet shrublands between 915-965 meters.