Polyscias sandwicensis is a glabrous tree that can reach up to 20(–30) meters, characterized by a straight trunk and a spreading crown. Most parts of the tree contain secretory canals, with the ends of the branches being thick, nearly fleshy, and densely tomentose. The leaves are compound and alternate, with the plant being deciduous during the dry season. Blades are broadly ovate to oblong or ovate-lanceolate, measuring 7–10 cm long, with juvenile leaves either simple or comprising 3–5 shallowly incised leaflets. Leaf apices range from acute to obtuse, with bases cordate to truncate. The upper surfaces of the leaflets are medium or yellowish green, while the lower surfaces are lighter and shiny. Leaf margins are crenate to serrate, with the petiole bases often broad and sheathing the stem. Stipules are absent. It has paniculate–racemose inflorescences that are either glabrous or sparsely stellate pubescent, with ultimate units being racemules. Peduncles are stout, ranging from (3–)5–30 cm in length, with spreading pedicels that are 2–10 mm long. Flower buds are initially greenish, often turning orangish purple before anthesis. Flowers are bisexual (perfect) and actinomorphic (radially symmetric), with a calyx presenting as a truncate, irregularly undulated rim. The corolla consists of 8–12 greenish-yellow petals, often tinged orange to purple externally, fleshy and caducous. Stamens equal the number of petals, with greenish-yellow filaments and dark yellow anthers. The fruits are globose drupes, dark purple, measuring 6–8 mm in diameter, with a flat-topped disk to low conical. Each drupe contains as many pyrenes as there are carpels, with seeds being singular per pyrene, possessing a small embryo and fleshy endosperm. It is found in dry to occasionally mesic forests between 30-800 meters.