Culms 15–75 cm. Leaves 2–4 per culm; sheaths with inner band hyaline, 3.5–12 cm, apex deeply concave, glabrous; ligules 1.7–7 mm, apex acute or often emarginate; blades plicate to flat, 9–50 cm × 0.8–3.9 mm, widest leaf 2.1–3.9 mm wide. Inflorescences 1.8–7 cm; spikes 4–8; lateral spikes pistillate often with few staminate flowers proximally, 2.8–10.8 mm, sessile, staminate portion (0–)1–6-flowered, (0–)0.8–4.6 mm, pistillate portion 4–25-flowered, 2–7.6 mm; basal 2 spikes 5–27 mm apart; terminal spike gynecadrous, 4.7–16.5 mm, staminate portion 3–13-flowered, 1.8–12.7 × 0.9–2.1 mm, pistillate portion 6–23-flowered, 2.8–7.3 × 4.4–6 mm. Pistillate scales ovate, 1.2–2.2 × 1–1.6 mm, apex acute. Staminate scales ovate, 1.7–3.3 × 1–1.4 mm, apex acute. Anthers 1–1.9 mm. Perigynia spreading to reflexed, green, 6–14-veined abaxially, 0–6-veined adaxially over achene, elliptic to elliptic-ovate, 1.8–2.9 × 1–1.9 mm, 1.2–2.1 times as long as wide; beak 0.2–0.6 mm, 0.09–0.33 length of body, smooth. Achenes ovate to orbiculate, 1.2–1.6 × 0.85–1.4 mm.

Fruiting late spring–early summer. Acidic, sandy, peaty hardwood or Chamaecyparis swamps, thickets; 0–300 m; Ont.; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ind., Ky., Md., Mass., Mich., Miss., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., S.C., Tenn., Va.

Carex seorsa very rarely hybridizes with C. atlantica subsp. atlantica.

Carex seorsa is unusual in sect. Stellulatae in its primarily forest understory habitat, lax, spreading habit, perigynia widest in the middle, and smooth perigynia beaks. Its affinities require further study.