Schiedea kauaiensis is an erect to ascending subshrub growing 30 to 100 cm tall. The stems, often swollen at the nodes, are terete, 4-angled or -ridged, few-branched, and glabrous, but they become sparsely and then moderately glandular-puberulent in the inflorescence, with internodes that are purple in color. A fibrous root system supports this plant. The leaves are simple and opposite, with oblong-elliptic blades measuring 7.5 to 15 cm long and 1.8 to 4.1 cm wide. The apex of the leaves is acute to weakly acuminate, and the base is often connate around the stem, gradually attenuate. The leaf surfaces are light green or yellowish green, with the upper surfaces slightly glossy and the lower surfaces slightly thickened and rubbery. The blades are chartaceous when dry, usually slightly undulate, with only the midvein evident, or sometimes with an additional pair of inconspicuous, smaller, looping veins. The margins are entire, slightly thickened, and weakly revolute, particularly toward the base. The midvein is slightly eccentric. The petioles are 0.5 to 1.1 cm long, pale green, and purple toward the base, with a weakly grooved texture. Stipules are absent. The inflorescence is terminal, with 27 to 70 flowers on a structure measuring 20 to 48 cm long. It is diffuse, with widely spaced flowers and branches progressively more densely puberulent towards the apex. The bracts are subulate, with the lowermost bracts of the central axis being elliptic-lanceolate, green like the leaves, recurved, and often twisted. The pedicels are (7–)10–23 mm long, elongating slightly in fruit and asymmetrically flattened. The flowers are bisexual (perfect), with 4.3 to 5.1 mm long sepals, lanceolate, green, opaque, and strongly reflexed. The corolla (petals) is absent. There are 10 stamens with dimorphic filaments and subequal pale yellow anthers. The superior ovary is 1-celled, with free-central ovule placentation and 3 styles. The fruit is an ovoid capsule measuring 3.1 to 3.5 mm long, with seeds approximately 1.3 mm long, orbicular-reniform, compressed. The perisperm (nutritive tissue surrounding the embryo) is starchy, with a hard or rarely soft consistency and no true endosperm. It is found in open areas of diverse mesic forests between 395-800 meters.