The relict, freshwater diatom genus Pliocaenicus Round & Håkansson contains several species from fossil and extant collections. In this study, we present LM and SEM observations of the main morphological characters of the species within the genus supplemented with data from the literature. The description of the genus is emended, based on ultrastructural observations of alveolae, rimoportulae, valve face and mantle fultoportulae and their position on the valve, presence of fenestrate structure on the valve face/ mantle junction, as well as presence of a ligula-like segment of the cingulum. We also provide a taxonomic key for species in Pliocaenicus. The biogeographic and palaeo-biogeographic distribution of the taxa in the genus from the Miocene till Recent is presented.