Cyanea longiflora is a terrestrial, unbranched shrub growing 1–3 meters tall. The stems are woody and muricate in juveniles, becoming smooth as they mature. The leaves are simple and alternate, with elliptic or oblanceolate blades measuring 30–55 cm in length and 6–12 cm in width, with an acute apex and glabrous surface, though muricate in juveniles. The leaf margins are entire or callose-crenulate, becoming irregularly cleft or lobed in juvenile plants, and are sessile or attached to 0.3–3 cm long petioles. The axillary inflorescence comprises 5–10 flowers featuring 30–60 mm long peduncles and 5–15 mm long pedicels. The hypanthium is obconic or cylindrical, 6–10 mm long, and glabrous. The calyx is 5-lobed, forming an irregularly toothed sheath, with the corolla being dark magenta, 6–9 cm long, and either glabrous or pubescent. The stamens are five in number, connate, with a glabrous, dark magenta staminal column and dithecal anthers that open by longitudinal slits and bear apical tufts of white hairs on the lower two. The plant produces obpyriform berries 10–12 mm long, containing numerous small, brown to black, smooth, and shiny seeds. These seeds are 0.5–0.7 mm long, with a straight, short to spatulate dicotyledonous embryo embedded in oily endosperm. It inhabits mesic to wet forests at elevations ranging from 275-760 meters.