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.