Herbs, perennial, cespitose, rhizomatous; base cormlike; rhizomes scaly, 12 cm × 2 mm. Culms trigonous, 20–55 cm × 1.2–11.8 mm, glabrous. Leaves V-shaped, 12–30 cm × 2–3 mm. Inflorescences: heads digitate, 12–20 mm diam.; rays 7–12, 1–13 cm; 2d order rays 1–3 cm (sometimes absent); bracts 3–5, longest ± erect, V-shaped, 5–12(–18) cm × 2–3 mm. Spikelets (8–)20–30, linear-lanceoloid, compressed, 5–10(–14) × 1.4–1.7 mm; floral scales (8–)16–26, laterally brown to reddish brown, medially green, laterally 1-ribbed, medially 3-ribbed, broadly elliptic, 2–2.5 × 1.2–1.6 mm, apex with slightly excurved cusp 0.2–0.3 mm. Flowers: stamens 3; anthers 1 mm, connective apex reddish, subulate, 0.1 mm; styles 1–1.5 mm; stigmas 1.5 mm. Achenes white to light brown, sessile, ellipsoid, 0.7–0.8 × 0.25–0.35 mm, apex obtuse, apiculate, surfaces puncticulate.

Fruiting early summer (May–Jun). Permanent pools and wet swales between sand dunes; of conservation concern; 1200 m; Tex.

Cyperus onerosus is apparently restricted to Andrews and Winkler counties in Texas.

This interesting endemic is most similar to Cyperus dentatus; it lacks tubers and apparently is not proliferous.