Stenogyne rugosa is a decumbent vine or erect herb, reaching heights of 2–7 dm and spreading by rhizomes. The stems are either cylindrical (terete) or four-angled, ranging from glabrous to densely tomentose. The leaves are simple and opposite, with blades that are ovate to elliptic or occasionally lanceolate, measuring 3.7–10 cm long and 1.9–4.2 cm wide. The apex is acute, and the base varies from acute, obtuse, or truncate to rarely somewhat cordate. The leaf surfaces may be glabrous or densely tomentose, and the blades are membranous. The margins are finely or coarsely serrate. The petioles are 5–19 mm long and moderately to densely tomentose. Stipules are absent. The flowers are arranged in verticillasters of 2–16(–24), axillary, and occasionally raised on peduncles 5–7 mm long. Pedicels range from 3–8 mm long and may be glabrous to densely tomentose. The calyx is campanulate, radially symmetrical, or occasionally bilabiate, 6–14 mm long, enlarging in fruit, and varies from glabrous to sparsely or densely tomentose. The teeth are linear-lanceolate, 1.5–8 mm long, 1.5–2.5 mm wide at the base, and split longitudinally, usually persisting or sometimes only the ventral half remaining. The corolla is bilabiate and varies from dark to dull red or green with brown-tinged lobes. It is densely tomentose externally, especially distally, and glabrous inside. The corolla tube is narrowly funnelform and straight, measuring 7–17 mm long. The upper lip is 3.5–6 mm long, entire or 2-lobed, and may be flat or concave. The lower lip is approximately 2 mm long, 3-lobed, with ovate lobes of similar size, which may have flat or revolute margins. The distal portion of the corolla is sparsely to densely pubescent externally, with simple or multicellular hairs, occasionally glandular pubescent within but otherwise glabrous. A nectary disk is often present at the base of the ovary, either annular or developed on the anterior side only. It has four stamens, with both pairs of similar length or slightly longer upper pair. The stamens may be included within the corolla or well-exserted beyond the upper lip. Filaments are inserted near the throat and are pubescent. The anthers are dithecal, with divaricate sacs that open by longitudinal slits. The ovary is superior and 2-carpellate, with each carpel longitudinally divided and containing one ovule per lobe with basal-axile placentation. The style is exserted, arising between the lobes, and usually cleft, diverging after anther dehiscence into two short, subulate lobes. The stigmas are two-lobed. The fruit is a drupaceous, one-seeded nutlet measuring approximately 3–5 mm long. The nutlets are black at maturity, with slightly winged margins and an obliquely attached base, enclosed within the calyx at maturity. The exocarp is fleshy, and the nutlets are obovoid. The seeds contain scanty or no endosperm with an oily texture. It is found in mesic forests, along the borders of lava flows at lower elevations, and in subalpine woodland between 500-3,100 meters.