Pittosporum glabrum is a shrub or small tree that grows 4–8 meters tall. Its young parts show a sparse to moderate or occasionally dense pale brown tomentose coverage. The stems feature sparsely brown tomentose branches, becoming glabrate over time. The leaves are simple and alternate with narrowly oblanceolate to obovate or spatulate blades, occasionally ovate, ranging from 4 to 23 cm in length and 1 to 7 cm in width. The leaf apex varies from acute or short-acuminate to obtuse or rounded, and the base is typically attenuated. The upper leaf surfaces are glabrate and sometimes retain patches of tomentose pubescence; the lower surfaces are glabrate but may show pale brown tomentose, especially in young leaves of plants in exposed areas, occasionally persisting on mature leaves. The leaves are chartaceous to subcoriaceous with entire margins that are slightly to strongly revolute and feature 9–24 pairs of lateral veins. Petioles range from 0.5 to 3.2 cm in length. The flowers occur in sparsely to moderately tomentose inflorescences, with peduncles 4–85 mm long and pedicels 7–20 mm long. The plant displays unisexual flowers, 1–20 in number, located in axillary or sometimes terminal, simple, corymbose clusters. The calyx is broadly cup-shaped with five distinct sepals measuring 2.5–7 mm long, covered in tomentulose to glabrate material with ciliate margins. The corolla comprises five white to cream or yellowish-white petals, each 8–21 mm long. It features five stamens alternating with the petals and a superior ovary with 2 or 3 carpels and 2–3 cells. The style is singular with a capitate stigma. The fruits are capsules, ovoid to subcuboid or somewhat compressed and oblong-ovoid, measuring 12–35 mm in length, initially brown tomentose, then glabrous to scattered brown tomentose as they mature. The capsules have a usually persistent style approximately 5–6 mm long, and the woody exocarp is 1–5 mm thick and deeply rugose or sulcate. The capsules contain about 12–35 black to reddish-black seeds that are sub-reniform and compressed to irregularly angled, each 6–9 mm long. It is found in mesic to wet forests and the margins of bogs between 24-1,970 meters.