Zanthoxylum kauaense is a small to medium-sized tree, ranging from 1 to 15 meters, with trunks up to 40–50 cm in diameter. The bark is brown to grayish-brown, and the new growth can be glabrous or finely puberulent, with young branches featuring a pair of bud scales. The leaves are compound and alternate, with 3 to 11 leaflets that are ovate to elliptic, sometimes broadly elliptic or even suborbicular, and thick. These leaflets range from 4 to 21 cm long and 2 to 14 cm wide, with lateral petiolules articulated only at the base and a terminal petiolule that may be once or twice articulated. The apex of the leaflets varies from obtuse to rounded or sometimes bluntly acute, with the base being cuneate to rounded. The leaf surfaces are glabrous or sometimes finely puberulent, especially when young, and are coriaceous with entire or minutely crenulate margins. The flowers appear in axillary, open, cymose inflorescences of usually 8 to 15 cm in length, containing 15 to 150 or more unisexual and actinomorphic flowers. The calyx consists of 4 to 5 deltate sepals, while the corolla comprises 4 to 5 white petals, sometimes tinged pink or red. There are 3 to 5 stamens, glabrous, and absent in pistillate flowers, leading to falcate, obovoid follicles that are 8 to 15 mm long, containing a single, black, subglobose, compressed seed 7-13 mm in diameter. It is predominantly found in mesic forests but sometimes in dry to wet forests, at elevations ranging from 300-1,980 meters.