Phyllostegia waimeae is a scandent subshrub species, with stems densely covered in retrorsely villous to retrorsely strigose hairs. The leaves are opposite, ovate, ranging from 5–13 cm in length and 2.5–6 cm in width, with acuminate apices and cordate bases. The leaf surfaces are notably villous, with hairs that are denser and longer along the veins and somewhat appressed on the upper surfaces, and feature very sparse glandular dots. The leaves have crenate margins and are supported by 1.2–5.5 cm long petioles, displaying hairiness like the stems. The flowers are six per verticillaster (an inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem) in open, unbranched, terminal, racemose inflorescences about 10–15(–20) cm long. The pedicels are short, 0.5–2 mm long, and villous. The bracts are broadly ovate, measuring 8–15 mm in length, and are also villous, with the lowermost pair being larger, at 17–50 mm in length. The calyx is narrowly obconical, measuring 4–6 mm at anthesis, and expands in fruit. It is covered in antrorsely villous and short-hispidulous hairs, with very sparse glandular dots, and typically has five teeth. The corolla is bilabiate, predominantly pale pink to pink, sometimes white, with an 8–12 mm long tube and sparsely villous. The stamens are four in number, ascend the upper corolla lip, and are included and aligned with the sinuses. The ovary is superior, 2-carpellate, with basal-axile placentation and one ovule per lobe. The style is short and bifurcated. The fruits are nutlets with seeds that lack an endosperm or are scanty and oily. It is found in diverse mesic to wet forests, typically at elevations between 915-1,100 meters.