The Kostomuksha State Nature Reserve (KSNR) is located in northwestern Russia, in the Republic of Karelia (hereinafter -Karelia), near the Finnish border. It has an area of 492.76 km 2 . This Protected Area was established in 1983 to protect a typical area in the north-taiga subzone of European Russia. The paper contains the results of a long-term inventory of mosses. We critically checked own samples from 1995-1998 and 2016, deposited into the Herbarium of the Karelian Research Centre of RAS (PTZ). Own samples, collected in 2010, 2012 and 2017-2019, have been processed. All available data, including literature, were summarised. An annotated list of 180 moss species was compiled. It makes up 70% of the moss flora of the Kuitozero-Leksozero floristic province and about 35% of Karelia's moss flora. Twenty-six species are reported for the KSNR for the first time. Of them, six species (Campyliadelphus elodes, Racomitrium fasciculare, Didymodon rigidulus, Eurhynchium angustirete, Grimmia elatior, Grimmia torquata) are new to the Kuitozero-Leksozero floristic province. The distribution of some known species was clarified based on the latest records. The taxonomic and ecological-coenotic structure of the moss flora has been revealed. Most moss species (75%) are widespread in the north-taiga subzone of Karelia. There are five species included in the Red Data Book of the Republic of Karelia (2020): Campyliadelphus elodes (3 VU), Eurhynchium angustirete (3 VU), Neckera pennata (3 NT), Diobelonella palustris [=Dichodontium palustre] (3 VU), Sphagnum auriculatum (3 VU). For the first three protected species KSNR is the northernmost locality in Karelia and for Diobelonella palustris it is the only locality in Karelia. Nine species (Callicladium haldanianum (NT), Campyliadelphus elodes (NT), Dicranum spadiceum (VU), Mnium lycopodioides (NT), Neckera pennata (VU), Schistostega pennata (VU), Sphagnum contortum (NT), Sphagnum inundatum (EN), and Sphagnum subnitens (NT)) are included in the Red List of Finnish species (2019). Seven species (Campyliadelphus elodes (NT), Fontinalis dalecarlica (NT), Helodium blandowii (NT), Meesia triquetra (NT), Scorpidium scorpioides (NT), Splachnum ampullaceum (NT), and Tomentypnum nitens (NT)) are listed on the European Red List (2019).