Phyllostegia mollis is a suberect perennial herb with densely short–hirtellous (somewhat antrorse hairs) stems. The leaves are simple, opposite, and range from ovate to occasionally elliptic-ovate, measuring (6–)10–24 cm in length and (2.5–)3.3–7 cm in width, with an acute to short-acuminate apex and a subcordate or occasionally truncate base. The upper surfaces are sparsely strigillose and usually short–hirtellous along the midvein, also moderately glandular-dotted. The lower surfaces are glandular-dotted and short-hirtellous, densely so along the veins, and the blades are flaccid. The margins of the leaves are evenly crenate. The flowers are (2–4)6 per verticillaster (inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem) in relatively open and delicate, compound, terminal racemose inflorescences measuring 8–17 cm long. The pedicels are 1.5–2(–3) mm long and finely short-hispidulous. The calyx is obconical, 3–4 mm long, enlarging in fruit, and short-hispidulous, usually with five linear-deltate teeth. The corolla is white and bilabiate with a slender, slightly curved tube 8.5–12 mm long. The fruits are nutlets with slightly winged margins approximately 2–3 mm long, 1-seeded, dark green to black, drupaceous, and obovoid. They emerge obliquely from the connate base and are spread and exserted from the calyx at maturity. The seeds have an absent or scanty, oily endosperm. It was found in diverse mesic to wet forests between 450–860 meters.