Five new species of Eleocharis are described based on specimens from South America: Eleocharis angustispicula, E. densicaespitosa, E. hatschbachii, E. ramboana and E. urceolatoides. Diagnostic characters, description, detailed illustration and taxonomic comments are provided. In addition, we propose the lectotypification of Eleocharis glauco-virens and a new synonym to E. loefgreniana. All those species belong to Eleocharis ser. Tenuissimae, which has a pantropical distribution with some species reaching temperate areas. A preliminary key to separate the species of the series that occur in Brazil is also presented.