Phyllostegia grandiflora is a vine to scandent subshrub with stems that are moderately to densely covered in backward-straight hairs (retrorsely strigose), particularly at the nodes and petiole bases but become smoother (glabrate) over time. It has simple, opposite leaves with blades that are ovate to narrowly ovate or elliptic–ovate, measuring between 7.5 and 16 cm in length and 3 to 6.4 cm in width. The leaf apex is short-acuminate, and the base ranges from cuneate to truncate. The upper surfaces of the leaves are sparsely to moderately strigose, while the lower surfaces are moderately to densely strigose, especially along the veins. The leaves are subcoriaceous with crenate–serrate margins, and the petioles, which are 1.5 to 5.5 cm long, are densely covered in forward-straight hairs (antrorsely strigose). The flowers appear in congested, unbranched, terminal, racemose inflorescences 10–15 cm long and elongate further when in fruit. Each verticillaster (inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem) contains six flowers on pedicels that are 6–12 mm long. The flowers are either perfect or unisexual, with the plants possibly being gynodioecious or rarely dioecious, and are usually fragrant. The calyx is obconical, 6–12 mm long, and covered in forward-straight hairs. The corolla is bilabiate, white with possible lavender tinges, and the tube is narrowly funnelform and slightly curved, measuring 15–25 mm in length. It has four stamens with dithecal anthers that open longitudinally. The superior ovary leads to a drupaceous fruit that is dark green to black, about 6 mm long, and one-seeded. The seeds lack an endosperm or are scanty and oily. It occurs near open, cloud-swept summits in mesic to wet forests between 300-1,230 meters.