Trematolobelia kauaiensis grows to a height of 0.9–4 meters. The stem measures 0.7–1.6 cm in diameter, glabrous, with cicatrices that are shallowly obdeltate, shallowly obtriangular, widely obtrullate, or narrowly transversely elliptic, measuring 1.7–3.8 mm tall and 2.5–6 mm wide. The leaves are simple and alternate, forming a dense spherical, oblate, or cylindrical apical rosette. The blades are elliptic or narrowly elliptic, measuring 12–30 cm long and 1.2–3.3 cm wide, with a length-to-width ratio of 6–13. The leaf apex is acuminate, and the base is attenuate, sometimes appearing petiole-like. The surfaces are glabrous, and the blades are coriaceous, with crenulate or subentire margins. Stipules are absent. The flowers are arranged in glabrous racemes, each with 20–32 flowers. The peduncle is 8–34 cm long, with sterile bracts that are elliptic or narrowly elliptic, measuring 18–100 mm long and 6–15 mm wide, with crenulate or subentire margins and obtuse, acute, or acuminate apex. The rachis is 10–44 cm long, with floral bracts that are lanceolate, elliptic, or broadly elliptic, measuring 8–35 mm long and 3–8 mm wide, with crenulate margins and obtuse, acute, or acuminate apex. The pedicels are 12–25 mm long, with oblong or narrowly oblong bracteoles, 1.5–6 mm long. The hypanthium is campanulate or obconic, measuring 5–8 mm long and 4–8 mm in diameter, 1/11–1/6 the length of the corolla. The calyx lobes are triangular and spreading, measuring 4–9 mm long and 1.5–3 mm wide, with entire margins and obtuse apex. The flowers are tetracyclic, bisexual (perfect), protandrous, zygomorphic (bilaterally symmetric), epigynous, pedicellate, resupinate, and ornithophilous. The calyx is synsepalous, with a glabrous tube adnate to the ovary, forming a campanulate or obconic hypanthium. The corolla is unilabiate and various shades of red, measuring 45–68 mm long. The tube is arcuate, measuring 20–40 mm long, 5.5–7.5 mm tall at the middle, and 3.5–6 mm at the mouth. The dorsal lobes measure 23–33 mm long and 1.5–2.5 mm wide, with an acuminate apex. The ventral lip is oblong, measuring 15–30 mm long and 6–8 mm wide. There are five stamens, antisepalous, connate distally for most of their length. The staminal column is strongly exserted, emerging well above the lobes. The filament tube is glabrous and is 45–61 mm long, 3.2–4.3 times longer than the dorsal anthers. The anther tube measures 3–4 mm in diameter and is pubescent toward the base. The dorsal anthers measure 12–16 mm long, and the ventral anthers measure 8–12 mm long, with tufts of white trichomes measuring 1.2–2.5 mm long at the apex. The ovary is inferior, 2-loculed, adnate to the hypanthium, and flat at the apex. The placentae are axile, with 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 fruit is a subglobose or hemispheric capsule, measuring 7–11 mm long, 7–14 mm wide, and 5–8 mm thick, with a few basal pores. The seeds measure 0.8-1.2 mm long. It is found in areas dominated by various lowland wet shrublands, montane wet shrublands, and lowland wet forests between 750–1570 meters.