Urtica sandwicensis is an erect annual herb reaching 20–60 cm in height. The stems are slender and usually branched, covered with stiff, erect stinging hairs (hispid) and short hirtellous hairs, especially dense in younger growth. The leaves are simple and opposite, with lanceolate to oblong-lanceolate blades measuring 1.5–7 cm in length and 0.9–2.5 cm in width. The leaf apex features an oblong terminal tooth with a rounded tip, while the base is truncate to broadly cuneate. Surfaces are covered in stinging hairs, and the margins are very coarsely serrate-incised. Each leaf has 3 to 5 prominent veins. Petioles are 0.8–2.5 cm long, and stipules are minute, oblong, lateral, and distinct. The flowers are arranged in sessile, axillary, cymose glomerules with no bracts, and both sexes are intermixed within the same cluster. Flowers are unisexual, actinomorphic (radially symmetric), and form dense clusters 0.2–0.5 cm long that are predominantly pistillate. The calyx of staminate flowers has 4 imbricate lobes, each about 0.5–0.6 mm long, and is sparsely pubescent with short, hooked hairs. Pistillate calyces are about 1 mm long, slightly swollen in fruit, evenly short-hispid with hooked hairs, and nearly closed at the apex. There are no petals. Staminate flowers have 4 stamens with incurved filaments that reflex elastically to release pollen; anthers are reniform and open by longitudinal slits. Pistillate flowers have a superior, 1-celled ovary (pseudomonomerous) with a basal ovule and a single style topped by a persistent, capitate-penicillate stigma. Staminate flowers may contain a vestigial, sterile ovary. The fruit is a compressed, ovoid achene about 1.1 mm long, containing a single seed. It is found in subalpine woodlands and alpine zone between 1,780-2,600 meters.