Yellow flatsedge is a tuft-forming sedge. Its stems are from 5 to 30 cm tall, 3-sided, and glabrous. There are 1-5 leaves, often lacking blades butsimply sheathing the stem, and 10-18 cm long. The infloresence consists of 1-3 spikes, each 1-3 cm long, each of these usually subtended by a bract (reduced leaf) 1-12 cm long. Each spike, in turn, contains 1-6 yellow to yellowish brown spikelets compressed and layered together like shingles. The flowers are subtended by straw-colored (stramineous) scales no more than twice as long as wide, and have 2 stigmas. The fruit (achenes) are hard, black to reddish-brown, lens-shaped, and 1-1.2 mm long.

Cyperus polystachyos and C. filicinus have narrower scales, about 3 times as long as wide.