Phyllostegia warshaueri is a scandent to decumbent lianas, with stems approximately 100–300 cm long and covered with antrorsely puberulent hairs. The leaves are simple and opposite, with oblong-elliptic to ovate or oblong-ovate blades measuring 7.5–15 cm in length and 2–6.6 cm in width, featuring an acute apex and cuneate base. The leaf surfaces are mostly glabrate, though the lower surfaces may have sparse puberulence along the margins or veins. The serrate leaf margins are occasionally remotely so, and the petioles, 2–5 cm long, are glabrous or sparsely haired. It produces 6–14 flowers per verticillaster (an inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem), arranged in unbranched, elongate, terminal, racemose inflorescences extending up to 20 cm, with 25–40 mm long pedicels that are primarily glabrous. The bracts are ovate to elliptic, measuring 7–21 mm, usually sparsely ciliate, with the lowermost pair larger, between 11–50 mm long. The calyx is narrowly obconical, around 6–8 mm long at anthesis, enlarging as the fruit matures, with five deltate teeth. The bilabiate corolla is white with a dark rose upper lip; the tube is narrowly funnelform, 18–20 mm long, and sparsely villous. Stamens, with the two pairs similar in length, are positioned to ascend the upper corolla lip. The fruits are nutlets approximately 6–7 mm long, with seeds that lack an endosperm or are scanty and oily. It is found in wet forests between 720-950 meters.