Myrsine kauaiensis is a small, branched shrub or tree that grows between 1–2.5(–4) m tall. The branches are pale brown, with younger ones appearing yellowish-green and glabrous. The leaves are simple, alternate, and have oblanceolate to narrowly elliptic or occasionally elliptic blades measuring (4.5–)5–9 cm in length and 2–2.5(–3.5) cm in width. The apex is acute to obtuse and may be minutely emarginate. The base is narrowly cuneate to subtruncate. The surfaces are glabrous, with blades yellowish green, younger ones appearing pale yellow or yellow with a reddish tinge, and are thinly fleshy. The margins are entire and scarcely revolute. The midrib is prominent, and a submarginal vein is evident, though the venation is inconspicuous. The petioles are 2–5 mm long. Stipules are absent. The flowers are arranged in bracteolate fascicles of 3–5, often arising on short woody knobs below and among the leaves, with glabrous pedicels measuring 8–14 mm long. The flowers appear to be bisexual (perfect). The calyx consists of 4–5(6) lobes, usually deeply divided, imbricate or valvate in bud, punctate or with secretory lines, and measuring about 0.5–0.7 mm long with sparsely ciliate margins. The petals are yellow with reddish margins, 4–5(6)-lobed, with lobes nearly distinct or connate up to ½ their length, imbricate in bud, narrowly oblanceolate, and 3–4 mm long, with prominent secretory lines and moderately ciliate margins. The anthers are approximately 1.6 mm long, with a tuft of hairs at the apex. The ovary is superior, 3–5(6)-carpellate, and 1-celled, with few ovules in a single row. In staminate flowers, the ovary is reduced, sterile, or absent. The style is short or absent, terminal, and the stigma is subsessile and capitate-angled. The fruit is a dark purple drupe, subglobose, and about 6–7 mm in diameter. Each fruit contains a small, dark-colored, subglobose seed excavated at the base. It is found in diverse mesic to wet forests between 850-1,250 meters.