A small diatom with a peculiar shape is often identified as Navicula schmassmannii in samples collected from alpine lakes and the Arctic region. Due to its characteristic outline, it is relatively well-identifiable, but scanning electron microscope (SEM) confirmation is essential for proving species identity. This species likely has a palaeoecological indicator value of warming climate in mountain lakes, although it is generally a minor component of the diatom assemblages. Here we re-investigate its type material to clarify its identity and taxonomic position. We provide an emended description based on SEM observation, including new information on girdle morphology. Furthermore, another population of N. schmassmannii was studied. Detailed morphological observations using light microscope (LM) and SEM were done on specimens from a sediment core obtained from Lake Brazi in the Southern Carpathian Mountains. Late-glacial and early Holocene populations of the diatom resembling N. schmassmannii showed high morphological variability in comparison with the type material. On the basis of details of type material and the different morphotypes from Lake Brazi, the transfer of N. schmassmannii Hustedt to the genus Humidophila is proposed as H. schmassmannii (Hustedt) Buczkó et Wojtal.