During the survey on the diatom diversity in mountain streams of the Gorce Mountains National Park (southern Poland), several unidentified taxa were observed. Comparison with available literature and detailed morphological analyses using light microscopy (LM) and scanning electron microscopy (SEM) confirmed a set of distinct valve features in one of the taxa that could not be assigned to any known taxa. As a result, Placoneis montana sp. nov. is described as a new species. A detailed morphological comparison with similar taxa is provided, along with information on the associated diatom assemblage. This new Placoneis species expands knowledge of morphological and ecological variation in the genus, and its lack of visible areola occlusions raises questions about the reliability of that character for separating Placoneis from Witkowskia, pointing to the need for further SEM-based comparison across related taxa. Additionally, observations based on the type locality suggest that the new species is associated with high-altitude, well-oxygenated, low-nutrient environments, emphasising their potential value as indicators of mountain stream conditions.