Stenogyne purpurea is a trailing or scandent vine, occasionally forming large mats. The stems are weakly 4-angled, with smaller branches covered in tomentose hairs. The leaves are simple, opposite, and elliptic, measuring 3.5–13 cm long and 1.2–4.9 cm wide. The apex is acute or attenuate, while the base is acute to truncate. The surfaces are glabrous or sparsely puberulent, and the blades are membranous. The margins range from serrate to subentire. The petioles are (4–)7–19 mm long and puberulent. Stipules are absent. The flowers are arranged in axillary verticillasters with 2–6(–16) flowers per cluster. The pedicels are 4–5 mm long and puberulent. The calyx is nearly radially symmetrical, narrowly campanulate, measuring 7–12(–15) mm long at anthesis, enlarging in fruit, and puberulent. The teeth are lanceolate, 2–5.5 mm long, 1.5–2.5 mm wide at the base, splitting longitudinally, usually persistent or sometimes only ventral 1/2 persistent. The corolla is bilabiate, with the tube and lobes pinkish purple or rarely white externally and the throat whitish. The corolla is tomentose externally and glandular-pubescent within. The tube is narrowly funnelform and straight, 15–22 mm long. The upper lip is 8–9 mm long, entire or 2-lobed, flat or concave, while the lower lip is 4–5 mm long and 3-lobed, with the lobes ovate, flat, or margins revolute. The distal portion of the corolla is sparsely to densely pubescent externally, with simple or multicellular hairs, and occasionally glandular-pubescent within. A nectary disk is often present at the base of the ovary, annular, or developed on the anterior side only. There are four stamens, with both pairs of similar length or slightly longer upper pair. The stamens are either included within the corolla or well-exserted beyond the upper lip. The filaments are inserted near the throat and pubescent. The anthers are dithecal, with divaricate anther sacs that open by longitudinal slits. The ovary is superior, 2-carpellate, with each carpel longitudinally divided, and the placentation is basal-axile. Each ovary lobe contains one ovule. The style is exserted, arising between the lobes, usually cleft, diverging after anther dehiscence, shortly 2-lobed, with subulate lobes. The stigmas are two in number. The fruit consists of dark purple nutlets measuring 7–11 mm long at maturity. The seeds have endosperm that is either absent or scanty and oily. It is found in diverse mesic and wet forest habitats between 600-1,300 meters.