Carex kauaiensis is a tufted perennial sedge with short, woody rhizomes. Its culms are trigonous, growing between 100–150 cm tall, and are smooth or weakly scabrous along the angles below the inflorescence. The leaves are simple and 3-ranked, with linear, plicate blades that range from 6–13 mm wide and are either shorter than or as long as the culm. The leaf apex tapers gradually to a long-acuminate point, and the margins are entire. The sheaths are reddish brown, with a chestnut brown hyaline ventral side that splits into parallel fibers. Cataphylls are brown to purplish brown, ventrally splitting into dark brown fibers. Stipules are absent. The flowers are arranged in 6–9 subdigitate spikes near the apex of the culm. The terminal spike is purplish brown, staminate, cylindrical, and 2–6 cm long, typically shorter than the lateral spikes. The lateral spikes are androgynous, with a short staminate apical part, cylindrical, 6–15 cm long and 3–6 mm wide, moderately open, and sometimes exposing rachilla internodes between the flowers. The spikes are subsessile, with the lowest occasionally bearing a short peduncle and ascending. There are 3–4 bracts, with the lower 1–2 much longer than the inflorescence and not sheathing. The pistillate glumes are dark purplish brown, membranous to chartaceous, elliptic or ovate, 4–5 mm long and 1.5–2 mm wide, 3-nerved, with the midnerve yellowish and prolonged into a serrulate–scabrous awn 1.5–2 mm long. The perigynium is subcoriaceous, dark purplish brown or blackish purple, glossy, ellipsoid to obovoid–ellipsoid, 3.5–6.5 mm long, and 1.5–2 mm wide, glabrous, inconspicuously 2-nerved, and abruptly narrowed to a short beak with a bidentate and usually spinulose orifice. The flowers are unisexual, with no corolla (petals). The ovary is superior, with two stigmas. The fruit is a broadly obovoid, lenticular achene. Each seed is enclosed within a thin seed coat free from the pericarp, with a basal embryo and abundant endosperm. It is found in open sites within wet forests between 1065-1225 meters.