Trematolobelia grandifolia grows to a height of 0.9–2 meters. The stem is 1.2–1.7 cm in diameter and glabrous, with depressed obtriangular cicatrices measuring 2–2.8 mm tall and 6–6.8 mm wide. The leaves are simple and alternate, forming a dense spherical, oblate, or cylindrical apical rosette. The blades are oblong or broadly oblanceolate, measuring 16–32 cm long and 2.7–6 cm wide, with a length-to-width ratio of 4.3–6.2. The leaf apex is obtuse, and the base is attenuate, sometimes appearing petiole-like. The leaf surfaces are glabrous and coriaceous, with crenulate or entire margins. Stipules are absent. The flowers are arranged in racemes, each with 12–20 flowers, and are glabrous. The peduncle is 16–25 cm long, with sterile bracts that are elliptic or oblong, measuring 25–88 mm long and 9–20 mm wide, with a crenulate margin and acute apex. The rachis is 14–30 cm long, with elliptic floral bracts, measuring 17–43 mm long and 6–10 mm wide, with denticulate margins toward the apex and an acute apex. The pedicels are 21–31 mm long, with oblong or broadly elliptic bracteoles, 3–7 mm long. The hypanthium is hemispheric or campanulate, 5–10 mm long and 7–12 mm in diameter, measuring 1/14–1/9 the length of the corolla. The flowers are tetracyclic, bisexual (perfect), protandrous, zygomorphic (bilaterally symmetric), epigynous, pedicellate, resupinate, and ornithophilous. The calyx lobes are triangular or lanceolate, erect, measuring 7–9 mm long and 2.2–4.5 mm wide, with entire margins and an obtuse or acute apex. The corolla is sub-bilabiate and white, measuring 65–75 mm long. The curved tube is 32–44 mm long, 7.5–9 mm tall in the middle, and 5–8.5 mm at the mouth. The dorsal lobes are 24–39 mm long and 1.5–2.5 mm wide, with an acuminate apex. The ventral lip is oblanceolate, measuring 23–38 mm long and 6–9 mm wide. There are five stamens, antisepalous, connate distally for most of their length. The staminal column is somewhat exserted, emerging above the dorsal lobes. The filament tube measures 55–62 mm long, 3.7–4.7 times longer than the dorsal anthers, and is pubescent in the upper half. The anther tube measures 3.2–4.2 mm in diameter and is pubescent at the base. The dorsal anthers measure 12–17 mm long, and the ventral anthers measure 10–13 mm long, with tufts of white trichomes measuring 1.5–2 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 an oblate capsule measuring 9–12 mm long, 12–13 mm wide, and 6–9 mm thick. Numerous pores are distributed over the entire lateral surface. It is found in various lowland wet shrublands, montane wet shrublands, and montane wet mixed communities between 650-1,280 meters.