Polyscias waimeae is a tree species reaching heights of 6–10(–12) meters, with secretory canals in most parts of the plant. Its leaves are compound and alternate, with blades ranging from 20–60(–75) cm long. The leaflets number between 5–11(13), ovate to oblong-elliptic, measuring 10–22 cm long and 4–8.5 cm wide, with the lowest pair often smaller and ovate. The leaflet apices are rounded-obtuse, sometimes slightly emarginate, while the bases are rounded to broadly cuneate, often oblique. The leaflet surfaces are glabrous, with coriaceous blades and entire margins. The leaves are petiolate, with petiolules measuring 15–22 mm long, and stipules are absent or rudimentary. The flowers are arranged in erect inflorescences, which are once compound-umbellate. The primary axis is short or absent, while the 3–4 secondary axes (rays) are stout and range from 5–20 cm long. The rays typically lack bract scars, though they may rarely have one, and each ends in a terminal umbellule of 8–20 flowers. The flowers are bisexual (perfect) and actinomorphic (radially symmetric), often appearing terminal, though they may appear lateral. Pedicels are thick and measure 15–65 mm long. The calyx is reduced to an undulate rim with poorly defined lobes. The corolla consists of 7–8 thick, triangular petals, which are pink or reddish and measure 9–12 mm long. Stamens are numerous, with 45–66 arranged in two whorls. The ovary is inferior, with (7)8–12 carpels, surmounted by a reddish, conical nectary disk. The stigmas are (7)8–12 in number and sessile. The fruit is a drupaceous structure that is globose–urceolate, ribbed when dry, and measures 20–25 mm long. The calyx forms an undulate crown at the top of the fruit. Each pyrene contains one seed with a small embryo. It is found in wet forests between 550-1,200 meters.