Clermontia tuberculata is a species of shrubs or trees that grow 4–5 meters tall, and feature light gray or light brown stems branched repeatedly and spread out, with a solid pith and white, viscous latex. The leaves are simple, alternate, with oblanceolate or oblong blades 9–20 cm long and 2.5–5.5 cm wide, with an obtuse apex and a cuneate or attenuate base. The upper surfaces are glabrous and dark green, while the lower surfaces are pale green and either glabrous or minutely muricate on the veins. The flowers are typically 1 to 2 per inflorescence, with muricate peduncles and pedicels that are 0.5–1 cm and 3–5 cm long, respectively. The flowers are bisexual, epigynous (having the ovary enclosed in the receptacle, with the stamens and other floral parts situated above), zygomorphic (bilaterally symmetric), resupinate, and protandrous, with a hypanthium that is 1.4–1.9 cm in length and diameter, and muricate in texture. The calyx lobes are triangular to deltate and muricate, with a bilabiate corolla, dark rose, and muricate measuring 5–7 cm long. The stamens are syngenesious (arrangement of stamens where their anthers are fused while their filaments are free), with dark purple anthers and a staminal column that is somewhat exserted. The ovary is inferior, with two carpels and two locules, leading to a slender, terete style topped with a two-lobed stigma that becomes receptive after pollen dispersal. The seeds are numerous, tiny, dark brown or black, with a minutely foveate-reticulate surface, containing a straight, short to spatulate dicotyledonous embryo embedded in oily endosperm. It is found on the windward slopes of Haleakalā, within montane rainforests and cloud forests at 1,650–1,825 meters.