Eleocharis laeviglumis is a new species from the Brazilian flora. This species belongs to Eleocharis subg. Limnochloa (P. Beauvois ex T. Lestiboudois) Torrey and is very similar to Eleocharis elongata Chapman. The new species differs from all species of Eleocharis R. Brown by culms terete, not septate; lower scale fertile; upper scales cartilaginous, smooth, without prominent veins, with submarginal band vinaceous to brown, hyaline margin conspicuous and abruptly differentiated from the rest of scale; slender bristles; achenes trigonous, reticulate, and without a short neck on the apex. Morphological description, illustration, data on habitat, and comparison with the similar species are presented.