Phyllostegia helleri is a weakly erect to scandent perennial herb. The stems are hirtellous, with somewhat crinkly but spreading hairs 0.5–1 mm long, occasionally retrorsely appressed. The leaves are simple and opposite, with ovate blades 10–14.5 cm long and 3.5–6.5 cm wide. The apex is long-acuminate, and the base is truncate to occasionally subcordate. Surfaces are hirtellous, with shiny, somewhat crinkly, unequal-length hairs, and may be inconspicuously glandular-dotted. The blades are thin and somewhat rugose. Margins are crenate–serrate or dentate–serrate. Petioles measure (2–)2.5–5 cm in length and are also hirtellous. Stipules are absent. The flowers are arranged in verticillasters of 4 to 6, within leafy, compound, terminal, bracteate racemose inflorescences that are usually 8–19 cm long, consisting of a primary branch and 2 or more lateral branches. Pedicels are 3–5(–9) mm long and hirtellous. Bracts are ovate to narrowly ovate, 3–12 mm long, with the lowermost pair reaching 15–30 mm; their pubescence resembles that of the leaves. The calyx is narrowly campanulate, 4–5 mm long, enlarging in fruit, and is hirtellous with inconspicuous veins and sparse glandular dots. It has five narrowly deltate teeth, 1–1.5 mm long, with acute apices. The corolla is bilabiate, white with lobes tinged lavender, and the tube is narrowly funnelform, curved, 9–12 mm long, and villous. The upper lobe is about 2.5–3 mm long, and the lower lobe is 5–7 mm long. A nectary disk is often present at the base of the ovary. There are four stamens ascending the upper corolla lip, included within the corolla. Filaments are inserted near the throat, and the anthers are dithecal with divaricate sacs. The fruits are nutlets approximately 2.5 mm long. Seeds contain little to no endosperm and are oily. It is found in diverse mesic to wet forest habitats between 860-1,536 meters.