Cyrtandra kauaiensis is a shrub that grows between 1.5 to 3 meters tall. Its stems are many-branched, rounded to angular, and can be either pubescent with multicellular hairs or glabrate. The leaves are simple, opposite, and borne on the upper three to five nodes. The leaves of a pair are unequal, symmetrical, or nearly so, and range from elliptic to elliptic-ovate in shape. They measure 5.5–15 cm long and 2.1–5.5(–6.3) cm wide. The apex is acute to acuminate, while the base is cuneate to attenuate. The upper surfaces are sparsely appressed hirtellous, whereas the lower surfaces are moderately to densely velvety pilose, especially along the veins. The blades are firm-chartaceous, and the margins range from entire to serrulate. The lower surface usually has conspicuously raised veins. The petioles are 0.9–2.3 cm long, sometimes with a base that clasps the stem or is perfoliate. Stipules are absent. The flowers occur singly (or rarely in pairs) in cymes that arise in the leaf axils and are densely appressed pilose. The peduncles measure 9–15 mm long, and the pedicels are 8–18 mm long. The bracts are linear, distinct, or occasionally connate, measuring 3–5 mm long. The flowers are bisexual (perfect). The calyx is nearly actinomorphic (radially symmetric), green, 10–16 mm long, and cleft nearly to the base, with oblong to linear-oblanceolate lobes. Both surfaces are moderately appressed to occasionally sparsely appressed pilose, with acuminate apices that are deciduous or persistent in fruit. The corolla is usually bilabiate and white, with a cylindrical, curved tube that measures approximately 13–15 mm in length and 5 mm in diameter. It is densely shaggy villous. The upper lip is two-lobed, while the lower lip is three-lobed. The lobes are imbricate in bud, with the upper lobes being broadly elliptic, around 7 mm long and 6 mm wide, while the lower lobes are broadly elliptic, 8–9 mm long, and 7–8 mm wide. The five stamens are inserted about halfway up the corolla tube, with the two upper stamens being fertile and the others staminodial with abortive anthers. The fertile anthers are coherent and positioned in the throat of the corolla tube, dithecal, opening by longitudinal slits or occasionally by apical pores. The ovary is superior, oblong-ovoid, two-carpellate, and one-celled, with parietal placentation. The placentas are two and ± intruded, rarely joined in the center to divide the ovary into two cells. The ovules are numerous, anatropous, pilose, or occasionally glabrous. The style measures about 5 mm long and is sparsely puberulent or occasionally glabrous. The stigma is capitate and two-lobed, with the lobes usually elliptic. The fruit is a fleshy or firm white berry, ellipsoid in shape, measuring 1–1.2 cm long. It is glabrous and often tipped by the persistent stylar beak. The seeds are numerous, ellipsoid, and minute, measuring 0.2–0.3 mm long, with a reticulate surface. It is found in diverse mesic forests between 610-1,220 meters.