Plants cespitose; rhizomes long, lax. Culms trigonous in cross section, 13-55 cm, scabrous distally. Leaves: basal sheaths reddish brown; ligules shorter than wide; blades mid to dark green, V-shaped, 2.5-5(-6.2) mm wide, glabrous. Inflorescences 2.5-10(-13) cm; proximal bract 0.6-19 cm, shorter than or equaling inflorescence; proximal 1-3 spikes pistillate, erect; terminal 1-3 spikes staminate. Pistillate scales ovate, 2.2-4.5 × 1-1.7 mm, as long as or shorter than perigynia, margins entire, apex obtuse or acute, awnless. Perigynia spreading or reflexed, often dark-colored, obscurely few-veined, veins not running into beak, elliptic, 2.2-4.1 × 1.3-2.2 mm, abruptly contracted; beak 0.2-0.8 mm, obscurely bidentulate, smooth, teeth straight, to 0.3 mm. Achenes yellow, trigonous, smooth. Stigmas 3. 2n = 74, 76-80.

Fruiting summer. Open wetlands, wet tundra, muskeg; 0-2000 m; B.C., Nfld. and Labr., N.W.T., Nunavut, Que., Yukon; Alaska; ne Asia.

Carex membranacea is common throughout arctic and high boreal regions of North America. It is noticeably absent from Greenland and occurs in Eurasia only in the Russian Far East (Chukotka and the Koryak mountains).

Morphologically Carex membranacea is closest to C. rotundata. Leaf and stem shape easily distinguish those two species but shape is often difficult to evaluate on herbarium specimens. Leaf sheath coloration, although not a definitive character, is helpful in distinguishing between the two species. Carex membranacea almost always has red basal sheaths; sheaths are usually brown in C. rotundata.