The LM and SEM morphology of nutlets of 10 taxa of the genus Eleocharis R.Br. (Cyperaceae: Cperoideae: Eleocharideae) was described from Tamil Nadu, India. The nutlets of the genus showed a range of diversity in terms of their colour (shiny blackish-purple, greyishbrown and yellowish-brown), shape (trigonous-obovoid to biconvex-obovoid), size (0.6–2.2 mm long), number of hypogynous bristles (3–10) and surface ornamentation (smooth, pitted, striate-reticulate and transversely ridged). A combination of nutlet features characterize each taxon and therefore, a taxonomic key based exclusively on the nutlet features was constructed to delimit the ten taxa of Eleocharis reported from Tamil Nadu, India.