Clermontia arborescens subsp. waikoluensis is a shrub or small tree, ranging in height from 1.5 to 8 meters, and can grow either terrestrially or epiphytically. The stems are repeatedly branched and spreading, light gray to brown, usually glabrous, though occasionally with sparse pubescence, and contain numerous helically arranged leaf scars near the apex. They also contain solid pith with viscous white latex. The leaves are simple and alternate, with oblong, elliptic, or oblanceolate blades that measure 10–19 cm in length and 3–5 cm in width. Leaf apices are typically acute to acuminate (sometimes cuspidate), and bases are usually cuneate or attenuate. The lower leaf surfaces are generally glabrous but may have sparse pubescence along the midvein. Margins are callose-crenulate, and petioles are 2–6 cm long. Stipules are absent. Inflorescences are axillary, most often 1-flowered due to abortion, but occasionally 2-flowered. They are glabrous or rarely pubescent, with peduncles 0.5–1.2 cm and pedicels 2.5–5.3 cm long. Flowers are perfect, zygomorphic (bilaterally symmetric), resupinate, and protandrous. The hypanthium is 1.5–2.1 cm long and glabrous. The calyx may be lobed (2–5 mm long), and the tube forms a hemispheric to obovoid hypanthium. Corolla length ranges from 5.8–9 cm, bilabiate in shape, green externally, and greenish-white or magenta internally. The trifid lip and dorsal lobes are well-developed, and corolla tubes and lobes may be pubescent or glabrous. The five syngenesious stamens form a curved column with dark purple anthers, and ventral anthers have white trichome tufts. The ovary is inferior and 2-loculed, with a terete style bearing a ring of stiff hairs near the apex and a 2-lobed stigma that becomes receptive after pollen dispersal. The fruits are orange berries, 2.5–4 cm long and wide, subglobose to obovoid, with a thick, leathery pericarp. They are crowned with a circular rim and the style base. Seeds are numerous, dark, shiny, and foveate-reticulate, containing a short, straight dicotyledonous embryo in oily endosperm. It is found in montane rainforests and cloud forests between 520–1280 meters on Molokaʻi and 600–1000 meters on Lānaʻi.