Pittosporum kauaiense is a small tree growing 4–10 meters tall, with young parts covered in pale brown strigose–tomentose hairs. The stems have branches that are whitish to pale brown, flocculose–strigose to subglabrate. The leaves are simple, alternate, and elliptic to broadly or occasionally narrowly obovate, measuring (4–)9–27 cm long and (2–)3.5–11 cm wide. The apex is short-acuminate to acute, while the base is attenuate. The upper surfaces have slightly impressed veins, at least sparsely whitish flocculose–strigose when young, and soon become glabrate. The lower surfaces have slightly raised veins, usually moderately whitish or occasionally brown, turning whitish flocculose–strigose, occasionally sparsely so. The blades are chartaceous to slightly coriaceous and moderately rugose. The margins are entire and sometimes slightly revolute. The lateral veins occur in 15–21 pairs. The petioles are 0.8–3 cm long. Stipules are absent. The flowers are arranged in inflorescences that are pale brown strigose–tomentose. The peduncles are 3–10(–20) mm long, with pedicels measuring 2–6 mm long. The bracts are subulate and 1–5 mm long. The flowers are unisexual, occurring in groups of 7–13 in axillary or cauline, simple, corymbose inflorescences. The calyx is cup-shaped, with five distinct sepals lanceolate to ovate, 2–3 mm long, pale brown tomentulose, with acute apices. The corolla consists of five white petals, 12–13 mm long. There are five stamens, alternating with the petals and distinct; the filaments are 5–6 mm long, while those of pistillate flowers are 3–4 mm long. The fertile anthers measure 1.5–2.3 mm long, while those of pistillate flowers are about 1 mm long. The ovary is superior, 2-carpellate, 2–3-celled, positioned on a short gynophore, with parietal placentation. The ovules are numerous and arranged in two rows per placenta. The style is singular, with a capitate stigma. The fruit is a capsule that is subglobose to subcuboid, measuring 9–20 mm long. It is brown or pale brown tomentose when immature and becomes glabrate at maturity. The apex is rounded to obtuse, with a persistent style measuring 2–4 mm long. The valves have a woody exocarp that is 2.5–3.5 mm thick, featuring a median furrow and a surface that is shallow to deeply rugose–tuberculate, with narrow troughs between the convolutions, sometimes obscured. The seed mass fills the capsule’s volume. There are about 10–18 seeds, which are black to brown, irregularly angled, and compressed, measuring 5–7 mm long. It is found in diverse mesic forests and valleys between 240-1,220 meters.