Phyllostegia hirsuta is an erect subshrub to scandent liana, with stems densely covered in long hirsute hairs that are often somewhat retrorse. The leaves are simple, opposite, and have ovate blades measuring 17–30 cm long and 7.3–18 cm wide. The leaf apex is acute to short-acuminate, and the base is subcordate to cordate. Both surfaces are moderately covered in long hirsute hairs, with the upper surfaces inconspicuously glandular-dotted and the lower surfaces more dense, making the blades rugose. The margins are crenate, and the petioles are 5.2–11 cm long, densely covered in long hirsute hairs that are often antrorse. Stipules are absent. The flowers are arranged in verticillasters of 6–18 flowers (with the uppermost verticillasters sometimes having only 4–8 flowers) in leafless, compound, racemose inflorescences usually 10–20 cm long. These inflorescences consist of a terminal primary branch and about 2 to several secondary, rarely tertiary, lateral branches. The pedicels are 1–5 mm long and long-hirsute, with ovate bracts 3–6 mm long. The calyx is obconical, 3–3.5 mm long, enlarging in fruit, hirsute at the base, less so above the middle, becoming sparsely short-hispidulous in the upper part, and glandular-dotted, often densely so. The calyx is usually 5-toothed, with ovate teeth that are 0.5–1 mm long, with an obtuse to rounded apex, splitting longitudinally and usually persistent or sometimes only the ventral half persistent. The corolla is bilabiate and white; the upper lip is typically purple. The tube is narrowly funnelform, slightly curved, 7–11 mm long, and sparsely to moderately villous. The upper lip is 2.5–3.5 mm long, entire or 2-lobed, flat or concave, and the lower lip is 5–7 mm long, 3-lobed, with ovate lateral lobes that are slightly asymmetrical and broadly ovate to suborbicular lower lobe that is slightly folded to form a groove, with somewhat undulate margins and an emarginate or entire apex. A nectary disk is often present at the base of the ovary, annular, or developed on the anterior side only. There are four stamens, both pairs of similar length or the upper pair slightly longer, ascending the upper corolla lip, included and aligned with the sinuses. The filaments are inserted near the throat and are usually pubescent. The anthers are dithecal, opening by longitudinal slits, with divaricate anther sacs. The ovary is superior and 2-carpellate, with one ovule per ovary lobe and basal-axile placentation. The style is included, arising between the lobes, shortly 2-lobed with clavate lobes, stigmatic only at the apex, with one lobe often rudimentary and two stigmas. The fruits are 2.5–3 mm long nutlets, 1-seeded, dark green to black, drupaceous, with a fleshy exocarp. The nutlets are obovoid with slightly winged margins, obliquely attached at the connate base, and spreading and exserted from the calyx at maturity. These seeds have endosperm that is absent or scanty and oily. It is found in diverse mesic to wet forests in the central Wai’anae Mountains and Ko’olau Mountains on O’ahu between 300-1,070 meters.