The revealed lichen diversity of Seskar Island has a total of 292 species, including 263 lichenized, 20 lichenicolous, and nine non-lichenized saprobic fungi. Stigmidium microcarpum is new to European Russia, Caloplaca alcarum, Cryptodiscus muriformis, C. pini, and C. tabularum are new to North-Western European Russia; Lecidella subviridis and Pyrenidium actinellum are new to the Le­ningrad Region. The most interesting result of the investigation is the discovery of the single modern population of the red-listed Flavocetraria nivalis in the Leningrad Region. The lichen biota of Seskar Island is moderately rich compared to other islands of the Gulf of Finland. Keywords: lichens, lichenicolous fungi, new records, protected species, North-West European Russia, State Nature Reserve “Vostok Finskogo Zaliva”.