Eugenia koolauensis is a small tree or shrub that grows to a height of 2–7 meters. The branches are densely covered with brown short-pilose hairs, which are somewhat appressed. The leaves are simple and opposite. Leaf blades are obovate to elliptic or suborbicular, concave, and measure 2–5 cm long and 1–3.3 cm wide. The apex is obtuse, apiculate, or slightly retuse, while the base is cuneate. The upper surfaces of young leaves are densely brown puberulent, becoming glossy and glabrous or retaining some brown puberulence near the veins as they age. Lower surfaces are minutely pitted or pustulate and densely brown puberulent, though occasionally moderately so. The leaf blades are coriaceous with strongly revolute margins. Leaves have 5–7 pairs of principal lateral veins, spaced 4–10 mm apart, with an irregular submarginal vein. Petioles are 1–4 mm long and covered in brown short-pilose hairs. Stipules are vestigial or absent. The flowers occur in clusters of 1–2 in the leaf axils, with pedicels measuring 1–8 mm long. Bracts are 1–2 mm long, brown, puberulent, and persistent. The flowers are bisexual (perfect) and actinomorphic (radially symmetric), with an obconical hypanthium 2–3 mm long, covered in brown short-pilose hairs. The calyx consists of 4 sepals, unequal in size, with one pair 3–4 mm long and the other pair 2–3 mm long, adnate to the annular nectary disk within the tube, and appressed puberulent near the base. The corolla consists of 4 white petals: ovate, obovate, or elliptic, 4–8 mm long, sparsely puberulent, and persistent. There are around 150 stamens, with white filaments ranging from 0.5 to 5 mm long, inserted on the nectary disk. The ovary is inferior, with axile ovule placentation and a short style. The berries are yellow to red, ovoid, and somewhat asymmetrical, measuring 0.8–2 cm long. The pericarp is fleshy, about 1–2 mm thick. Seeds are typically solitary (occasionally 2), globose, 4–9 mm in diameter, coherent to the pericarp, and distinct from the connate to distinct cotyledons. It was formerly found in dry gulches and slopes in the northern Ko’olau Mountains of O’ahu, ranging from Pupukea to Kaipapa’u, with two collections from Mauna Loa, Moloka’i between 100–300 meters.