Cyperus sandwicensis is a perennial plant with short, woody rhizomes covered with purplish-brown or reddish-brown fibrous remnants. Its trigonous, smooth culms stand 30–130 cm tall and 5–13 mm in diameter at the base. The leaves are simple and numerous, with linear, coriaceous blades 7–20 mm wide, ranging from shorter to as long as the culm, and purplish-brown cataphylls. The sheaths are reddish-brown or reddish-purple, and stipules are absent. The plant features large, umbelliform, compound inflorescences, somewhat lax, 10–25 cm long and 20–35 cm wide, with 10–15 spreading rays, each bearing 1–8 spikes. The spikes are oblong to cylindrical, 2–8 cm long and 1–2.5 cm wide, bearing numerous spikelets with bracteoles shorter than the spikes. Involucral bracts number 5–8 are significantly longer than the inflorescence. Spikelets are 4–8-flowered, linear, 6–13 mm long, and 1–1.5 mm wide, with reddish-brown, membranous glumes. The flowers are perfect (bisexual) without a corolla, have three stamens, and a superior ovary. The achenes are linear-oblong, trigonous, about 2.5-2.8 mm long, and 0.5-0.7 mm wide, each producing a single seed with a thin coat and abundant endosperm. This plant is typically found in open wet forests on cliffs, steep slopes, and valley floors between 90-1,160 meters.