Leptotrema lithophila, the rare species being recognized as endemic for Sikhote-Alin’ Mountain (Russian Far East), has been discovered for the South Siberia from the Transbaikal Territory. The species was found in a single locality – in the Alkhanai National Park, on isolated cliff named Dimchig Sume Mt. within the lower forest belt in rock’s cleft in wet and shade micro condition. All known localities of Leptotrema lithophila are given including the first locality of the species from Khabarovsk Territory. The species is excluded from the lichen flora of the Kamchatka Territory. A comparison of the material from the main and peripheric parts of its distribution area, as well as ecology and illustrations for the species are given. In addition, the red-listed species Coccocarpia erythroxyli and C. palmicola are reported for the first time for Alkhanai National Park. Those species, as well as Leptotrema lithophila, are treated as relics of Paleogene-Neogene thermophilic mesogydrophilic flora. Taking into account the relict status of this rare species, Leptotrema lithophila is recommended for inclusion in the Red Data Book of Russia.