Hillebrandia sandwicensis is an erect perennial herb standing 50–150 cm tall, emerging from shallow, yellowish, tuberous rhizomes. The stems range from glabrous to sparsely pubescent, adorned with long reddish-brown hairs approximately 2 mm long. Its leaves are simple, alternate, and 2-ranked in caulescent species, becoming many-ranked in acaulescent (lacking a stem), with asymmetrical, suborbicular blades about 10–25 cm in diameter. The leaf surfaces are hispidulous, featuring short, white, papillose hairs, particularly along the veins on the lower side, and have irregularly 5–9-lobed, serrate margins. Petioles measure 10–22 cm long, complemented by ovate–cordate stipules of 1.2–3 cm long, which remain attached. The flowers are in axillary inflorescences with 10–30 cm long peduncles, bracts in bright pink to white, cordate to ovate, and 0.8–1.5 cm long. The unisexual flowers, often somewhat irregular, are each subtended by showy, colored bracts, with staminate and pistillate flowers showcasing bright pink to white perianths. The staminate flowers feature ovate sepals, while pistillate flowers have ovate–lanceolate sepal lobes, with petals spatulate in both flower types. Staminate flowers possess numerous yellow stamens with anthers that open by two lateral slits. The partly inferior ovary is wingless, 1-celled with an imperfect 5-cell structure at the lower part due to placental intrusion, and has parietal ovule placentation. It has five distinct, bifid styles with spiral stigmatic lines on their branches. The fruits are capsules that measure 0.8–1.3 cm in diameter, with pale reddish-brown, oblong seeds roughly 0.5 mm long. It is found in open or shady wet ravines within wet forests between 600–1,850 meters.