Herbs, perennial; rhizomes 3-20 cm × 1-2 cm, indurate. Culms stoutly trigonous, thickened, 50-300 cm × 5-30 mm, basally indurate, glabrous. Leaves V-shaped, with cross ribs, 40-130 cm × 10-15 mm, margins and keel scabrid. Inflorescences: spikes appressed-ascending, narrowly cylindric, (2-)3-45 mm × (4-)7-10 mm; rays (5-)9-12, 2-22(-30) cm; 2d order rays distinctly flattened, 1-10 cm; 3d order rays flattened, 0.5-3 cm; rachis 2.5-4 cm; bracts (6-)9-10, ascending at 45-75°, V- or inversely W-shaped, (5-)15-90 cm × 0.5-20 mm; rachilla persistent, wings 0.2-0.4 mm wide. Spikelets (4-)10-35, appressed-ascending, compressed, linear, 7-12(-15) × 0.6-1.2 mm; floral scales deciduous, 6-20, marginally clear, laterally brown, medially green, laterally 2-3-ribbed, medially 1-ribbed, oblong-ovate, (3.2-) 3.8-4.3 × 1.2-1.7 mm, apex spreading, acute to obtuse, mucronulate. Flowers: anthers 0.7-0.9 mm, connective apex bright red, oblong, to 0.1 mm; styles 0.8-1.3 mm; stigmas 2-3.3 mm. Achenes brown, sessile, narrowly ellipsoid, 1.8-2.5 × 0.8-1.3 mm, apex ± acute, not apiculate, surfaces puncticulate.

Fruiting summer. Coastal marshes; 0-10 m; La.; Mexico; Central America; South America.

Cyperus prolixus produces large plants, similar in habit to C. giganteus and C. papyrus but easily distinguished by flattened rays.