Phyllostegia floribunda is an erect subshrub with 30–90 cm long, quadrangular, and hirsute stems. The leaves are simple, opposite, and range from ovate to lanceolate or elliptic, measuring 12–24 cm in length and 4–8.4 cm in width, with an acuminate apex, broadly cuneate to rounded base, and moderately hirsute surfaces that are sparsely glandular-dotted. The lower surfaces are pale and membranous. Margins are crenate, and petioles are hirsute, measuring 2.6–6.5 cm long. It features two flowers per verticillaster (inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem). It is located on short, leafless, congested lateral branches 4–12 cm long, forming racemose inflorescences. These branches are hirsute throughout and emerge from the axils or below the lowermost leaves, sometimes with a pair of reduced leaves at the base. Each flower is subtended by an obovate, ciliate bract 3–8 mm long. The calyx is broadly obconical, hirsute, with gland-tipped hairs and glandular-dotted, splitting longitudinally. The corolla is bilabiate, maroon, pink, or red, with a villous tube about 8–10 mm long, and has a bilobed upper lip and a 3-lobed lower lip. It has four stamens, a superior ovary with basal-axile placentation, and a shortly 2-lobed style. The fruit is a drupaceous, 1-seeded nutlet, 3-3.5 mm long, dark green to black, with a fleshy exocarp, and becomes exserted from the calyx at maturity. The seeds lack an endosperm or are scanty and oily. It is found in mesic to wet forests between 430-1,130 meters.