Herbs, perennial, rhizomatous. Culms trigonous, (10-)20-50 cm × (0.6-)1-2 mm, glabrous or more often scabridulous, either in distal 1/2 or over entire length; basal sheaths, 1-3, light brown, 1-4 cm, papery. Leaves flat to V-shaped, (2-)20-35 cm × 2-6 mm, margins and keel minutely scabridulous; proximal leaves bladeless. Inflorescences: spikes ± umbellate to slenderly (infrequently broadly) ovoid, 10-25 × 7-16 mm; rays 3-5, 1.5-8(-15) cm; bracts 3-7, erect (or declined at most 25-30° from vertical), (2.5-)8-20(-25) cm × (0.5-)1.5-6 mm; rachilla ± deciduous, wingless. Spikelets (1-)5-10(-20), oblong to linear-oblong, quadrangular, 7-10(-18) × (2.8-)3.2-4(-4.5) mm; floral scales deciduous, (1-)5-10(-14), spreading, laterally stramineous or dull whitish, sometimes also red-spotted, medially green, laterally 2-3-ribbed, medially 3-ribbed, broadly ovate to oblong-ovate, (2.3-)2.8-3.2 × (2-)2.4-3.2 mm, apex broadly rounded, often clear-erose, cusp 0.1-0.4 mm in proximal scales, 0.3-1 mm in distal scales. Flowers: anthers 0.8-1.4 mm; styles 0.1-0.3 mm; stigmas 1.5-3 mm. Achenes light to dark brown or nearly black, broadly ellipsoid, infrequently obovoid, 2-2.4 × 0.9-1.4 mm, base cuneate to ± stipelike, apex obtuse to broadly rounded, surfaces papillose. 2n = 166. Fruiting late spring-early summer. Riverbanks, sand bars, lakeshores, sand dunes, sandy openings in woods; 0-1000 m; Alta., Man., Ont., Que., Sask.; Colo., Ill., Iowa, Ky., Mass., Mich., Minn., Mo., Mont., N.J., N.Mex., N.Y., Ohio, Okla., Pa., S.Dak., Tex., Utah, Wash., Wis., Wyo.; Mexico (Coahuila). Cyperus schweinitzii is introduced, but not naturalized, in Massachusetts and Washington.