Herbs, perennial, rhizomatous, coarse. Culms trigonous, 150–250 cm × 2–15 mm, glabrous. Leaves inversely W-shaped, 30–150 cm × 6–22 mm. Inflorescences: spikes 1–4, cylindric, 1.5–4 cm × 7–15 mm; rays 3–9, 10–26 cm; 2d order rays 2–11 cm; 3d order rays 1–4 cm (or absent); bracts 3–5, ± horizontal, 20–100 cm × 3–15 mm; 2d order bracts 2–7 cm × 2–7 mm; 3d order bracts 2–5 cm × 2–4 mm (or absent); rachilla persistent, wingless or wing hyaline, narrow, 0.1 mm wide. Spikelets 50–100, linear, ± quadrangular, slightly compressed, 8–11 × 0.9–1.2 mm; floral scales 15–30, marginally clear, laterally reddish along midrib, medially green, laterally 3–4 ribbed, medially 3–5-ribbed, ovate, 1–2–1.6 × 1.1–1.3 mm, apex mucronulate. Flowers: anthers 2, 0.4–0.5 mm; styles 0.8–1 mm; stigmas 2, 0.4 mm. Achenes dark brown, sessile, ellipsoid to obovoid, 0.9 × 0.4–0.5 mm, surfaces finely puncticulate.

Fruiting summer. Pond shores, stream banks; 0–30 m; introduced; Fla.; West Indies; Asia; Africa; Australia.

Cyperus alopecuroides has been introduced in Polk County, Florida.

Plants of Cyperus alopecuroides might be confused with C. digitatus; large size and the biconvex achenes with two stigmas characterize C. alopecuroides.