Rhizomes 1.5-5 mm diam. Culms sharply trigonous, 0.5-1.5 m × 2-4 mm. Leaves 3-4, basal, smooth; sheath fronts not pinnate-fibrillose; blades 1 or 2, thickly V-shaped in cross section, to equaling sheath length. Inflorescences 1-2 timesbranched or subcapitate or of 1 spikelet, branches to 4 cm; proximal bract usually erect, trigonous, 2-7 cm. Spikelets 1-35, in clusters or solitary, 5-12 × 3-4 mm; scales straw-colored to orange-brown or midrib greenish, usually clearly lineolate-spotted, ovate, 3-3.5 × 2.5 mm, flanks ribless, midrib proximally spinulose, apex acute to rounded, notch 0.3 mm deep, awn 0.3 mm, sparsely spinulose. Flowers: perianth members 4-6, brown, bristlelike, equaling to less than 1/2 of achene body, retrorsely spinulose; anthers 2 mm; crest short, spinulose; styles 2-fid. Achenes brown, biconvex, ovoid to obovoid, 2-2.5 × 1.5-2 mm; beak 0.1-0.2 mm. 2n = 42.
In North America Schoenoplectus triqueter is known only from the tidal Columbia River system (B. W. Lightcap and A. E. Schuyler 1984), where it forms fertile hybrids with S. tabernaemontani [S. ×kuekenthalianus (Junge) D. H. Kent = Scirpus ×scheuchzeri Brugg]. Fertile hybrids between the same species also occur in Europe.