Perennial. Culms slender, straggling, ascending from a prostrate base, 20–50 cm tall. Leaf sheaths usually densely tuberculate-hairy, less often glabrous; leaf blades lanceolate to narrowly ovate, 1–15 × 0.3–3 cm, glabrous or variously hairy, base subrounded and usually suboblique, apex acute; ligule ca. 1 mm. Inflorescence axis 9–15 cm, glabrous or hispidulous; racemes 4–9, reduced to dense cuneate fascicles less than 2 cm long, the rachis often setose. Spikelets in 3–5 clustered pairs, lanceolate, hispidulous; glumes herbaceous, awned, the awns stout, purple, viscid; lower glume 3–5-veined, awn 5–10(–15) mm; upper glume 5-veined, awn 2–5 mm; lower lemma herbaceous, 5–9-veined, apex with a stout 1–2 mm mucro, palea absent; upper lemma subcoriaceous, smooth. Fl. and fr. Jul–Nov. 2n = 54.

Oplismenus undulatifolius is sometimes considered synonymous with O. hirtellus (Linnaeus) P. Beauvois. Although the two taxa intergrade, O. hirtellus (O. aemulus (R. Brown) Roemer & Schultes) generally has longer racemes (to 3 cm) of contiguous spikelets, at least in the lower part of the inflorescence. It has a more tropical distribution than O. undulatifolius and has recently been reported from Taiwan and Yunnan.

Light shade in forests, moist places. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hebei, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Yunnan, Zhejiang [warm-temperate and subtropical regions of the N hemisphere, uplands of India and Africa].