Zanthoxylum oahuense is a small tree 3–6 meters tall with thin gray to dark brown bark that is smooth to slightly furrowed; new growth is finely puberulent, becoming glabrate, and paired bud scales subtend the young branches. The leaves are alternate and compound with three coriaceous, broadly deltate leaflets; the lateral pair is usually asymmetrical (7–16 × 4.5–9 cm) and the terminal leaflet is similar; margins are minutely crenulate to subentire and blades are glabrous with obscure glandular punctae. Petiolules are distinctly articulated: laterals twice (at base and near leaflet) and 22–65 mm long; the terminal petiolule is articulated three times (at base, about two-thirds its length, and just below the leaflet) and 35–72 mm long; apices are acuminate-caudate, often falcate; bases ± asymmetrically truncate to subcordate; lateral veins (5–10 pairs) are strongly ascending with reticulate higher venation; the rachis is terete or winged; stipules absent. Inflorescences are axillary, open cymes with 15–40+ flowers, 8–12 cm long, finely puberulent; peduncles 35–85 mm, pedicels 3–5 mm, bracteoles <1 mm. Flowers are unisexual, actinomorphic (radially symmetric); calyx of 4(5) deltate, ~1 mm ciliolate sepals; corolla of 4(5) pale greenish-yellow, oblong-lanceolate, 4–7 mm petals; stamens 4(5) in staminate flowers (absent in pistillate); ovary superior, 1-carpellate with axile ovule; style oblique, stigma capitate. The fruit is a single falcate, obovoid follicle 9–13 mm long with a rugulose, pitted exocarp, containing one black, subglobose–obovoid, compressed seed 8–10 mm in diameter on a slender funicle. It is found in mesic to somewhat wet forest between 580–800 meters.