We describe as new to science six new species of the diatom genus Gomphonema Ehrenberg from Lake Baikal, based on light and scanning electron microscope observations. The six species are morphologically similar to G. ventricosum, a northern European taxon known mostly from Scotland, Scandinavia and other Baltic countries, though it has been recognized also from North America. The six species are similar to G. ventricosum W. Gregory in the structure of the external occlusions over the areolae. The species also have prominent septa and pseudosepta, which distinguishes them from other species of the genus. They are distinguished from one another in the shapes and sizes of the valves. This group of closely related species may correspond to a species flock, a phenomenon in freshwater diatoms that is known from ancient lakes and flowing water systems. Further research is needed to confirm the monophyly of this group, and either support or challenge the idea of them representing a species flock.