Pittosporum hawaiiense is a tree typically reaching 3–4 meters high, with young parts covered in a dense, brown to reddish-brown tomentose layer. The branches feature a similar tomentose covering that can become more glabrous as they age. The leaves are simple and alternate, with blades that are broadly oblong–oblanceolate to oblong–obovate, ranging from 15 to 28 cm in length and 3 to 8 cm in width, occasionally extending up to 36 cm by 10 cm. These leaves have an acute to short-acuminate apex and an attenuate base. The upper surfaces are somewhat glossy and rugose due to impressed veins; while initially densely tomentose, the undersides may become less so over time. The blades are chartaceous to subcoriaceous, and the margins are entire and slightly revolute. Lateral veins, numbering between 9 and 30 pairs, and tertiary veins are prominently reticulate towards the margins. The petioles, measuring 2 to 4 cm long, are sometimes narrowly winged in the upper half. The flowers occur in dense, brown to reddish-brown tomentose inflorescences with stout peduncles and pedicels. It bears 8 to 10 unisexual flowers in axillary, simple, corymbose arrangements. The calyx is cup-shaped with five distinct sepals, and the corolla consists of five cream-colored petals, each 13 to 18 mm long. The stamens are distinct and alternate with the petals. The ovary is superior, typically 2-carpellate and 2-3-celled, with a capitate stigma atop a single style. The fruit is a cuboid-ovoid capsule, 24–38 mm long, initially tomentose but becoming glabrate upon maturation. The capsule’s apex ranges from rounded to emarginate and features a persistent style. The woody exocarp of the valves is 3–8 mm thick and coarsely tuberculate. Inside, the seeds are approximately 30 in number, black to reddish-black, subreniform (somewhat kidney-shaped) to irregularly angled, and approximately 7–9 mm long. It is typically found in mesic to wet forests between 520-1,830 meters.