The flora of Chernihiv Polissya (northern Ukraine) is rich in rare taxa of desmids. One of these taxa is Cosmarium perforatum var. skujae Grönblad, the eighth documented occurrence of this taxon in Europe. Based on observations over several years, the morphology and taxonomy of C. perforatum var. skujae is discussed comparing it to the type variety C. perforatum var. perforatum living in the same habitat. Distinct morphological characters and differences in variability between these closely affiliated taxa are grounds for elevating C. perforatum var. skujae to the status of an independent species. In order to avoid homonymy with Cosmarium skujae Willi Krieger & Gerloff, a new name, Cosmarium tutum is proposed for this taxon.