Slender sedge is a graminoid, 20-80cm tall, with 3-5 spikes per culm, the lower spikes pistillate on erect to ascending peduncles, the terminal spikelet is staminate and peduncled, usually elevated above pistillate spikelets. It has 4-12 closely overlapping perigynia per spike, that are aggregated, ascending, conspicuously veined, elliptic-obovate, broadly rounded and spindle or top-shaped, the beak is abruptly bent, achenes are obovoid. The widest leaves are usually 5mm, the basal sheaths are reddish-purple or purple tinged, sheaths are 2-42mm, blades are ascending or lax, erect, green or yellow-green, not glaucous, midrib developed adaxially, and 2 lateral veins abaxially. The blades of overwintering leaves are smooth. The pistillate scales' apex is acute to aristate, awn to 1mm. The staminate scales' margins are hyaline, purple-brown or brownish purple tinged, and its apex acute.
It is most similar to Carex blanda, which has leaves with white bases, staminate spikelet that is sessile and shorter than the tips of the bracts, and somewhat longer perigynia with a shorter beak.