The diatom genus Pliocaenicus, considered a freshwater relict, is composed of several species. The genus is extremely diverse in the Pliocene and so far reported only from the Northern Hemisphere. Two known modern representatives of Pliocaenicus have been reported exclusively from Asian arctic and alpine zones (P. costatus and P. seczkinae). The morphology and key ultrastructural characters of all known species will be discussed. The morphological variability within Pliocaenicus costatus sensu lato and the closely related P. seczkinae indicate a need for defining new criteria for varieties and/or species level. Comparative analysis within taxa in Pliocaenicus provides a taxonomic key for species determination. Furthermore, the genus description could be emended for observations on seven more morphological features that help to clearly separate it from closely related genera. These observations will help determine the phylogenetic position of Pliocaenicus within the ThalassiosiraceaeStephanodiscaceae.The biogeographic distribution reflects the rich diversity of the genus Pliocaenicus during the Pliocene in Eurasia and the Holocene expansion of P. costatus group in the Asian Arctic and the Baikal Rift Zone Lakes. The intercontinental or endemic distribution pattern of particular species in the genus is noted. The biogeographic observations provide important implications for future palaeogeographical studies.