The new and rare small-celled centric diatom species Pantocsekiella teletskoyensis Genkal et Mitrofanova was recently identified from Cyclotella delicatula Genkal, previously described in the phytoplankton of Lake Teletskoye (Altai, Russia). The identification of P. teletskoyensis was based on the analysis of numerous scanning electron microscopy images since the beginning of a recent phytoplankton study in the lake. The investigation of the species revealed a wider variability of its qualitative morphological features such as valve relief, presence of granules on valve face, rimoportula position, and structure of alveoli. As a result, new data on the morphological variation of this species allowed for the refinement of its diagnosis. In addition, a comparison of the morphological features of P. teletskoyensis and other small-celled centric diatoms of Pantocsekiella and Cyclotella genera found in various water bodies across Europe, Asia, and America showed that P. teletskoyensis is most similar to C. minuscula, that was previously described from the deep-water Lake Ohrid located on the border of Northern Macedonia and Albania. We also conducted numerous measurements of P. teletskoyensis valve diameters, which enabled us to estimate the cell size in the population ranging from 3.02 up to 7.98 µm. The study of P. teletskoyensis population structure revealed that the group of 4.00-4.99 µm was consistently the most numerous among size groups with a step of 1 µm in all the months studied.