Coast sedge is a graminoid, 20-70cm tall, with a solitary terminal spike, occasionally with 1-3 smaller accessory spikes, gynecandrous with male portion 1-3mm wide, up to 50-flowered, female portion 5-9.5mm wide with up to 27 flowers. Plants are sometimes unisexual with male and female spikes on different plants. Its spreading to reflexed perigynia are castaneous to dark brown, 15-veined abaxially, faintly 7-veined adaxially, and lanceolate-ovate to broadly ovate. Beak is soft, serrulate with blunt teeth, achenes are ovate. It has 2-6 leaves per culm, sheaths are tight, inner band hyaline, apex concave, and glabrous. Blades are involute, the widest leaf 0.8-1.5mm wide.
Distinguishing characteristics include its densely cespitose habit, its (usually) single terminal spike, widely spreading flattened perigynia with a strongly serrulate beaks, and involute leaves that are 0.8-1.5mm wide.