Three new species of Eleocharis R. Br. subg. Scirpidium (Nees) Kukkonen (Cyperaceae) from southern Brazil are described and illustrated: E. angustirostris R. Trevis. & Boldrini, E. neesii R. Trevis. & Boldrini, and E. riograndensis R. Trevis. & Boldrini. They have fertile proximal scales and achenes with longitudinal rows of horizontally oriented rectangular cells, characteristic of Eleocharis subg. Scirpidium. Eleocharis angustirostris is distinguished from all the Brazilian species of the subgenus by the achene narrowing into a neck and the narrow stylopodium that appears to be a continuation of the neck; E. neesii is related to E. radicans (Poir.) Kunth from which it differs in having capillary culms and an inflated apex of the distal sheath. Eleocharis riograndensis is distinguished from all the Brazilian species of Eleocharis subg. Scirpidium by a combination of features including capillary culms, the long acuminate distal sheath apex, and two stamens with short anthers. A key is presented to separate the species of Eleocharis subg. Scirpidium known to occur in Brazil.