Cyanea platyphylla is a shrub that grows 1–3 meters tall. Its stems are unbranched and range from smooth to muricate or aculeate, with juveniles often densely aculeate. The leaves are simple and alternate, with a dimorphic structure: juvenile leaves are oblong to obovate, measuring 9–21 cm long and 4–7.5 cm wide, while adult leaves are oblong to oblanceolate or obovate, spanning 28–68 cm in length and 7–20 cm in width. The juvenile leaves have an acute to acuminate apex and a rounded to obtuse base featuring aculeate upper surfaces and pubescent, aculeate lower surfaces. Adult leaves have an acute to acuminate apex and a cuneate base, with glabrous or sparsely muricate to minutely aculeate upper surfaces and pale green, glabrous or sparsely pubescent to sparsely muricate lower surfaces. The leaf margins range from callose–dentate to lobed in juveniles and callose–denticulate to callose–dentate in adults. Juvenile petioles are 1.5–4 cm long and may be glabrous or pubescent and aculeate, whereas adult petioles are 6–19 cm long and glabrous or sparsely muricate. It bears 6–25–flowered inflorescences with 20–90 mm long peduncles and 10–25 mm long pedicels. The flowers are bisexual (perfect), with an obconical hypanthium measuring 5–9 mm in length and 3–5 mm in width. The calyx lobes are narrowly oblong to narrowly triangular, 3–7 mm long and 1–2 mm wide, with an acuminate apex. The corolla is white, yellowish-white with magenta stripes, or magenta, 42–54 mm long and 5–10 mm wide, and can be glabrous or pubescent. The corolla tube is arcuate with spreading lobes that are ¼–½ the length of the tube. It features five connate stamens that alternate with the corolla lobes. The staminal column is glabrous, and the anthers are glabrous, with the lower two having apical tufts of white hairs. The ovary is inferior and 2-celled, with axile ovule placentation. The stigma is 2-lobed, wet or dry, and becomes receptive after pushing the pollen through the anther tube. The fruits are pale orange ellipsoid berries 8–10 mm long and 6–8 mm wide, containing numerous seeds. It is found in lowland montane wet forests of Waipi’o Valley, windward Mauna Kea, Puna District, and windward Mauna Loa on Hawai’i island from 615-1,082 meters.