Phyllostegia macrophylla is a sprawling perennial herb or vine with dense stems that occasionally sparsely cover backward-pointing hairs (retrorsely strigose). It features simple, opposite leaves that are ovate to broadly ovate, measuring 7.5–15.5 cm in length and 3–8 cm in width. The leaves have an acuminate apex and a broadly cuneate to truncate base, sometimes slightly heart-shaped (subcordate). The leaf surfaces are sparsely to moderately covered in long, thin hairs, especially along the midvein on the lower surface. The leaf margins are crenate-serrate, and the petioles are 1.4–4 cm long, exhibiting a dense to sparse forward-pointing hairiness. Flowers appear 6–10 per cluster, occasionally up to 12 in the lowermost clusters, within unbranched, terminal, racemose inflorescences 10–20 cm long. The pedicels are 3–8 mm long, with variable pubescence, including a mix of forward and backward-pointing hairs. The calyx is broadly cone-shaped, 3–6 mm long, expanding in fruit, and features five broadly triangular teeth. The corolla is bilabiate, white to pink, with a narrow funnel-shaped tube around 7–12 mm long, densely covered in forward-pointing hairs. It has four stamens, with the ovary at the top (superior), leading to nutlets that are about 2.5–4 mm long, dark green to black, one-seeded, and have a fleshy outer layer. The seeds lack an endosperm or are scanty and oily. It occurs in wet forests between 750–1,230 meters, occasionally reaching up to 1,900 meters.