Cyanea spathulata is a branched shrub reaching 2 to 3 meters in height. Its foliage is composed of simple, alternate leaves with narrowly oblanceolate to narrowly elliptic blades measuring 8–22 cm long and 1–3 cm wide. The leaf apex is typically obtuse to acute, while the base is attenuate, either tapering into a winged petiole or forming a narrowly cuneate base with a non-winged petiole. The upper surfaces of the leaves are green and glabrous, and the lower surfaces are pale green, either glabrous or sparsely pubescent. Leaf margins are minutely callose–crenulate and often revolute. Petioles range from 1.5 to 3.5 cm long when winged and from 5.5 to 12 cm when non-winged. Stipules are absent.
The flowers are bisexual (perfect), arranged in inflorescences of 8 to 10 flowers. The rachis is either condensed or absent, with peduncles measuring 15–50 mm in length and pedicels measuring 8–15 mm in length. The hypanthium is broadly ellipsoid to broadly obovoid, measuring 2–3 mm in length and 1.5–2.5 mm in width. Calyx lobes are dentiform, 0.5–1 mm long, with an acute to acuminate apex. The corolla is white or greenish-white, sometimes tinged with pale purplish, 15–30 mm long and 1.5–3 mm wide, glabrous, with a suberect tube and spreading lobes. There are five stamens, alternate with the corolla lobes, united into a glabrous staminal column. Anthers are glabrous, though the lower two possess tufts of white hairs at their apex. The ovary is inferior and two-celled, with axile ovule placentation. The stigma is bilobed and remains appressed and nonreceptive during pollen dispersal, becoming receptive only after the style pushes through the anther tube. The fruit is a purple, globose berry measuring 5–8 mm in diameter and contains numerous seeds. It is found in diverse mesic to wet forests on western and south-central Kaua‘i between 700-1,200 meters.