Herbs, annual, cespitose. Culms trigonous, (5–)20–60 cm × (0.2–) 0.6–2.4 mm, glabrous. Leaves (1–)3–4, V-shaped, 15–40 cm × 3–6 mm. Inflorescences: spikes 2–5(–8), oblong to ovoid, 2–10 cm × 3–12 mm; rays 4–8, to 11 cm; bracts (4–)5(–7), ascending at 45°, V-shaped, (1.5–)8–30(–50) cm × 1–6 mm; rachilla persistent, wingless (successive floral scales 0.7–0.9 mm apart). Spikelets 10–30, compressed, linear to ellipsoid, 4–20 × 1.5–2.1 mm; floral scales deciduous, (2–)6–26, laterally brown to golden brown, edges clear, medially green, 5-ribbed, outermost ribs more noticeable than others, obovate to nearly orbiculate, 1.3–1.8 × 1.2–1.8 mm, margins emarginate, apex with mucro 0.05–0.12 mm. Flowers: stamens 2(–3); anthers (0.2–)0.3–0.4 mm, connective tips reddish, less than 0.1 mm; styles less than 0.1 mm; stigmas 0.2–0.4 mm. Achenes brown, slightly stipitate, obovoid, 1.2–1.4 × 0.5–0.7 mm, apex apiculate, surfaces puncticulate.

Fruiting summer–fall (Jul–Oct). Damp to moderately dry, disturbed soils; 0–500 m; introduced; Ala., Ark., Calif., Conn., Fla., Ga., Ill., La., Md., Miss., Mo., N.Y., N.C., Okla., S.C., Tenn., Tex., Va., W.Va.; Mexico; Central America; South America; Asia; Africa; Australia.

Cyperus iria was first recorded in the New World from the southeastern United States in the 1840s.