Phyllostegia stachyoides is a weakly erect to scandent subshrub with antrorsely strigose stems, occasionally with some hairs somewhat spreading. The leaves are simple and opposite, with lanceolate to ovate or occasionally broadly ovate blades measuring 6–17(–20) cm long and 2.2–6(–8) cm wide. The leaf apex is acute to acuminate, and the base ranges from cuneate to occasionally truncate. The leaf surfaces are moderately to sparsely strigose, denser along the veins, sometimes with spreading hairs, rarely glabrate, and usually moderately glandular-dotted. The blades are somewhat rugose, and the margins are crenate to shallowly crenate. The flowers, numbering (8–)10–14 per verticillaster (inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem), are part of usually compound, racemose inflorescences, consisting of the principal stem and typically two lateral branches immediately below. The pedicels are 2–7 mm long and vary from antrorsely strigose to antrorsely spreading hispidulous, or on Hawai’i island; they are villous. The calyx is campanulate, measuring 3.5–8 mm long, and enlarges in fruit, displaying a conspicuous glandular-dotted appearance, usually with five teeth. The corolla is bilabiate and white, with the upper lip often tinged pink. The tube is narrowly funnelform, about 6–12 mm long. The plant has four stamens, with both pairs of similar length or slightly longer upper pairs. The ovary is superior, 2-carpellate, with basal-axile placentation. The fruit consists of nutlets about 3 mm long, 1-seeded, dark green to black, and drupaceous. The seeds lack an endosperm or are scanty and oily. It occurs in mesic to wet forests between 880-1,400 meters.