Phyllostegia brevidens is a scandent subshrub with glabrate stems. Its leaves are simple and opposite and range from ovate-elliptic to elliptic–obovate. They measure approximately 7–13 cm in length and 2.5–5.3 cm in width, with an acuminate apex and broadly cuneate base. The surfaces are glabrous to sparsely strigillose, and the margins are dentate–serrate. Petioles are about 1.5–2.3 cm long, and stipules are absent. It features 14–20 flowers per verticillaster, organized in open, unbranched, terminal, bracteate, racemose inflorescences that extend to about 10–15 cm in length, with pedicels 10–16 mm long on a common peduncle of 1–2 mm. Bracts are approximately 20–60 mm long and very sparsely strigillose. The calyx is obconical, 4–6 mm long, enlarges in fruit, sparsely strigillose along the nerves, and minutely glandular–dotted, usually with five nearly obsolete teeth. The corolla is bilabiate and white, with a narrow funnelform tube generally curved, measuring about 8–12 mm long. The upper lip is entire or 2-lobed and flat or concave, around 6 mm long. In contrast, the lower lip is 3-lobed with lateral lobes ovate and slightly asymmetrical, and the lower lobe broadly ovate to suborbicular, about 10 mm long. It has four stamens with similar length pairs or the upper pair somewhat longer, and a superior ovary, 2-carpellate, with basal-axile placentation and usually one ovule per ovary lobe. The fruit consists of one-seeded nutlets, approximately 6 mm long, dark green to black, drupaceous with a fleshy exocarp and obovoid shape with slightly winged margins. The seeds lack endosperm or are scanty and oily. It is known from only two collections, both made before 1871, from Hilo, Hawai‘i, and the type from the forest of Mauna Kea at 915 meters.