Eugenia koolauensis, a member of the Myrtaceae (myrtle) family, is a small tree or shrub between 2 and 7 m (7 and 23 ft) tall with branch tips covered with dense brown hairs. The leathery, oval or elliptic leaves, 2 to 5 cm (0.8 to 2 in) long and 1 to 3.3 cm (0.4 to 1.3 in) wide, are densely hairy on the lower surface and have margins that curve under the leaves. One or two flowers grow from the leaf axils on stalks 1 to 8 mm (0.04 to 0.3 in) long. The hypanthium (basal portion of the flower) is cone-shaped, about 3 mm (0.1 in) long and hairy. The four sepals of unequal length that comprise the hypanthium are attached to a circular nectary disk (fleshy, nectar-producing structure). The four white petals, which are oval or elliptic and 4 to 8 mm (0.2 to 0.3 in) long, enclose numerous white stamens and are also attached to the nectary disk. The fruits are fleshy, yellow to red, oval berries 0.8 to 2 cm (0.3 to 0.8 in) long and contain one round seed.