This note describes a new record of Gomphonema firmum Skvortzow, which expanded both morphological and distributional ranges of the species. This conspicuous diatom was previously unknown outside of Lake Baikal, where it inhabits deep-water areas. The newly discovered locality belongs to a small stream at a distance of 180 km west from Lake Baikal, where the species was found in a relatively high abundance that enabled to trace a size diminution series including specimens shorter than previously described. In addition, G. lanceolatum var. maximum Poretzky, a name for a diatom from Lake Teletskoe, was included in a synonymy of G. firmum that further expanded the species distribution. In the light of the presented data, the species appears to be a rare diatom with current distribution limited to a few scattered localities in southern Siberia.