A new freshwater diatom, Fragilaria flexura sp. nov., was identified from light and scanning electron micrographs of sediment core material from a mountain lake in the Sredinny Mountains of Kamchatka Peninsula (Russia). Fragilaria flexura sp. nov. possesses an unique combination of morphological features, which allows its discrimination from previously identified, similar taxa such as Fragilaria cyclopum and Hannaea arcus. Fragilaria flexura sp. nov. is characterized by its valve outline, which is sickle-shaped and may or may not have a notch-like depression on the ventral margin, narrowly rounded ends and a very large, bilateral, hyaline central area. Fragilaria flexura sp. nov. seems to prefer meso-oligotrophic habitats.