Carex rotundata is an obligate wetland sedge with round stems and large, shiny black inflated perigynia. This species occurs in boreal zone wetlands, especially Sphagnum bogs and pond margins in peatlands, also occasional in marshes and fens on both sides of the Alaska Range in Denali. Plants form small clumps, with slender rounded stems 10-40 cm tall. The leaves are basal, as long as or longer than the stems, narrow (1-3 mm wide) and filiform. Plants produce male and female flowers in separate spikes. The terminal spikes are staminate, the lateral spikes pistillate. The pistillate spikes are widely spaced and sessile, or with a shot, erect peduncle. The pistillate scales are ovate, brown to black, with or with an acute beak, shorter and narrower than the perigynia. The perigynia are inflated, shiny dark brown or black, membranaceous, with a bidentate beak. The fruits are three-angled yellow achenes with3 stigmas. This species looks highly similar to Carex membranacea, but that species has angled stems, not rounded.

Carex rotundata fruits July to August.

Carex rotundata is monoecious, but male and female flowers are produced in separate spikes. Species in the genus Carex are wind pollinated, but usually also self-pollinate to some degree (Bertin 2007; Friedman and Barrett 2009). Seeds are wind and water dispersed.

Carex rotundata is found in wet places; muskegs, bogs, wet meadows.

Wet sites in the lowlands.