Clermontia oblongifolia subsp. mauiensis is a shrub or tree that grows 2 to 7 meters tall and is found in terrestrial and epiphytic environments. It has light gray or brown stems repeatedly branched and spread, with a solid pith and white, viscous latex. Its leaves are simple and alternate, with elliptic blades measuring 12–20 cm in length and 3–5.4 cm in width, featuring dark green, glossy upper surfaces and very pale green, dull lower surfaces. The inflorescences can be either glabrous or pubescent, with peduncles and pedicels that are 1.6–2.8 cm and 2.2–4.2 cm long, respectively. The flowers are bisexual, epigynous, zygomorphic (bilaterally symmetric), resupinate, and protandrous, with a hypanthium that is 1.4–1.8 cm long. The calyx is synsepalous, forming a hemispheric structure to obovoid, with five green, valvate lobes that mimic the shape and texture of the corolla. The perianth is 5.4–6.4 cm long, with an acuminate apex in bud and 1.8–2.5 cm long lobes. The stamens are syngenesious, with purple anthers that open introrsely, and the ovary is inferior, featuring two carpels and two locules with a large axile placentation. The style is slender and terete, leading to a two-lobed stigma that becomes receptive after pollen dispersal. The numerous tiny, dark brown or black seeds are minutely foveate-reticulate and contain a straight, short to spatulate dicotyledonous embryo embedded in oily endosperm. It is found in montane rain forests at 800-990 meters.