Clermontia hanaulaensis is a shrub or small tree that can grow up to 3 meters tall and typically flowers at heights between 1.5 and 3 meters. The stems branch near the base, and the bark is light brown and rugose-fissured, with green leafy branches with short internodes. It produces white latex. The leaves are alternate and clustered at the distal ends of the branches. The leaf blades are narrowly elliptic to oblanceolate, measuring 10–12(–18) cm long and 2.0–3.5(–4) cm wide, and are chartaceous. The apex is acute to short-acuminate, sometimes with a short mucro up to 1 mm long, and the base is cuneate to attenuate. The adaxial (upper) surface is green, glossy when fresh but drying to a dull finish, and is glabrous. The abaxial (lower) surface is paler than the adaxial surface and glabrous, with 15–16 secondary veins on each side. The leaves are petiolate. The inflorescences are (2)3–4(–5)-flowered and glabrous, with a peduncle measuring 15–30(–42) mm long. The bracts are triangular, about 1 mm long, and deciduous. The calyx is 1/2–4/5 as long as the petals, with petaloid lobes similar in color to the corolla. These lobes are often pale greenish tinged toward the base, sometimes creamy white with violet-purple irregular veins, and are connate for 1/3–1/2 their length, being erect to spreading rather than appressed to the petals. The corolla is weakly zygomorphic (bilaterally symmetric) to nearly rotate when fresh, slightly to moderately curved, with the perianth (30–)35–45(–50) mm long. The perianth tube is 15–25(–27) mm long and 9–10 mm wide, with lobes that are 20–26 mm long and (2.0–)3.0–3.5 mm wide. The dorsal and ventral lobes spread in the distal half and are pale violet-purple, often suffused with creamy white streaks, sometimes creamy white, and glabrous. The staminal column is violet, 2.0–2.5 mm wide, with filaments 30–40 mm long, darker violet anthers, and an anther tube 9–12 mm long and 2.3–3.0 mm wide. The fruit is a fleshy berry, dull orange in color, obconic to turbinate in shape, measuring 15–20 mm long and 10–15 mm wide, with a smooth surface. The sepals and petals are caducous. The seeds are obovoid, slightly compressed, 0.5–0.6 mm long, and 0.5 mm wide, with a dark brown, glossy, and smooth testa. It is found in montane wet forests, at approximately 1158–1213 meters.