Plants perennial; rhizomes 1-1.5 mm thick, soft, longer internodes 2-3 cm, scales 5-14 mm, tubers absent. Culms obscurely trigonous to terete; spikelet-bearing culms 16-80 cm × 0.5-1.5 mm; when submersed plants often forming numerous, filiform flaccid culms without spikelets, sometimes with whorls of slender branches, 0.1-0.3 mm wide, soft; sometimes septate-nodulose when aquatic, internally spongy, transverse septa incomplete. Leaves: distal leaf sheaths persistent or decaying, membranous, apex acute, often prolonged into translucent portion to 1 mm. Spikelets not proliferous, (6-)9-24 × 1.4-2.2 mm; rachilla joints bearing prominent winglike remnants of floral scales; proximal scale with a flower, amplexicaulous, 2.5-4.1 mm; floral scales 7-26, 1-2 per mm of rachilla, green to stramineous or pale brown, often minutely dotted reddish, usually with conspicuous dark brown to blackish submarginal band, narrowly ovate, 3.5-4.5 × 2 mm, thickly papery, membranous toward margins. Flowers: perianth bristles 6-7, whitish to stramineous or pale reddish brown, proximally slightly flattened, unequal, exceeding or rarely shorter than achene, 0.7-1.9 mm, retrorsely spinulose; anthers yellow to reddish, 1.7-1.9 mm; styles 3-fid. Achenes whitish, stramineous, or pale green, obovoid to obpyriform, compressed trigonous with adaxial face broadest, or biconvex, 0.65-1.4 × 0.5-0.8 mm, clearly sculptured at 10-15X, each face with 10-13 rows of rectangular, transversely elongated cells, apex constricted to short neck 0.2-0.25(-0.3) mm wide, wider at tubercle base. Tubercles dark brown, pyramidal, 0.2-0.5 × 0.2-0.4 mm.

Fruiting late spring-late fall. Sometimes drying ponds, lakeshores, marshes, creeks, canals, ditches; 10-50 m; Ala., Fla., N.C., Tex.; Mexico; Central America; South America.

Eleocharis elongata sometimes grows with E. robbinsii; no intermediates are known.