The diatom genus Entomoneis includes species with a unique morphology, having panduriform frustules in girdle view, often twisted about its apical axis, with some unique characters such as a sigmoid raphe on elevated bilobate keel, differently shaped junction lines, numerous girdle bands and variously perforated striae. Most of the species belonging to this genus are found to be brackish or marine, with some occurrences in freshwater habitats, but not one species has been described as exclusively freshwater. In this study, the new freshwater diatom species Entomoneis triundulata sp. nov. is described based on a detailed morphological observation using light and scanning electron microscopy. The specific features of E. triundulata are its triundulate valve margin; the shape of the transition between keel and valve body having a proximal U-shaped end, a slightly curved middle segment and a distal V-shaped end; and five sub-compartments to the frustule cavity. Besides morphological features unique to E. triundulata, similarities with other Entomoneis species are reported and discussed.