Cyanea undulata is an unbranched shrub that stands 1.8–3.6 meters tall. The leaves are simple and alternate, with narrowly elliptic blades measuring 28–36 cm in length and 3–5 cm in width. These leaves feature an acuminate apex and an attenuate base, with glabrous upper and pubescent lower surfaces. The margins of the leaves are undulate, and the plant has winged petioles that are 3–5 cm long, seamlessly extending from the base. It produces 5–6 flowers in axillary racemes that form pubescent inflorescences, with peduncles approximately 43 mm long and pedicels around 10 mm long. The hypanthium is oblong and pubescent. The flowers are bisexual (perfect) with a 5-lobed calyx whose triangular lobes are about 4 mm long with an acute apex. The corolla is yellowish, slightly curved, and pubescent, with spreading lobes. The flower structure features a suberect to a strongly arcuate tube dorsally cleft to about the middle. There are five stamens, which alternate with the corolla lobes and connate with a pubescent staminal column. The anthers are glabrous and dithecal, opening by longitudinal slits and coherent but separating after anthesis or connate and forming a tube into which pollen is shed. The ovary is inferior, 2-celled, with axile ovule placentation and a 2-lobed stigma that is wet or dry, appressed, and nonreceptive until the growing style pushes out the pollen. The fruit is a yellow obovoid berry, approximately 17 mm long and 11 mm wide, containing numerous small seeds that are brown to black, smooth, shiny, and 0.5–0.6 mm long. These seeds have a straight, short to, spatulate dicotyledonous embryo embedded in oily endosperm. It grows in narrow drainages and wet streambanks in dry to montane wet forest or shrubland, 670–790 m in elevation.