Phyllostegia renovans is an erect subshrub in its youth, which becomes scandent as it matures. It can reach lengths of up to 3–4 meters, with short-hirsute stems. The leaves are simple and opposite, with blades that vary from narrowly to broadly ovate, measuring 12.5-20 cm in length and 2.0-8.8 cm in width. The leaf apex is acuminate, and the base ranges from truncate to broadly rounded or broadly cuneate. The surfaces are covered in spreading hirsute hairs, primarily concentrated along the veins on the lower surface. The leaves often exhibit red coloration, especially in sunny locations, which appears less intense in shaded areas. The margins are coarsely dentate or serrate-dentate, and the veins are red-tinged. Petioles are reddish green, 3.2-5 cm long, and moderately to densely hirsute. Stipules are absent. Flowers appear in groups of (4)6 per verticillaster (inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem), sometimes with the terminal one hosting eight flowers, arranged in an open, unbranched, racemose inflorescence spanning 18-34 cm. The inflorescence is unique as the stem resumes vegetative growth post-flowering. Pedicels are 9-10 mm long and may rest on a common peduncle up to 4 mm long, featuring moderate to dense short-hirsute pubescence. The bracts are ovate, with lower ones measuring 6-11 cm and upper ones 2.5-3 cm long. The flowers are typically bisexual (perfect), though occasionally unisexual, suggesting a gynodioecious or rarely dioecious nature, with a light fragrance noticeable in the late afternoon, hinting at moth pollination. The calyx is campanulate, 8–12 mm long, enlarging in fruit, covered in short hirsute hairs, with lobes 4-7 mm long, displaying several coarse teeth and an acute apex. The corolla is bilabiate and white, approximately 19–22 mm long, with a narrow funnelform tube that is curved and short-hirsute, especially on the upper side. The lower lip is 9-12 mm long and 3-lobed. There are four stamens, with the upper pair sometimes slightly longer inserted near the throat, usually pubescent. The ovary is superior, 2-carpellate, with one ovule per ovary lobe, and a style that is shortly 2-lobed. The fruits are nutlets around 8-9 mm long, dark green to black, drupaceous, and obovoid, with a fleshy exocarp and slightly winged margins. The seeds lack an endosperm or are scanty and oily. It is known only from the upper parts of three valleys, typically growing along watercourses and at the bases of waterfalls between 680-1,040 meters.