The rare and poorly known Northern Hemisphere old‐growth forest lichen Arthonia incarnata is reported for the first time for Asia from Japan and Korea. A detailed description and illustration is provided for the species based on the collections from Japan and South Korea. Bayesian and RAxML analyses of mtSSU, nLSU and RPB2 sequence data show A. incarnata to be only distantly related to the other Arthonia species hitherto sequenced and in particular to superficially similar species occurring in the same type of habitat such as A. didyma, A. spadicea and A. vinosa . For the analysis we generated new sequence data of A. incarnata for the mtSSU (5 specimens) and RPB2 (5) gene loci. Slight variation was observed in sequence data of the RPB2 gene between collections from eastern Asia and Sweden, but this is not substantiated by differences in morphology or ecological behaviour. Arthonia incarnata is confined to humid forests and only grows on parts of tree trunks shaded from precipitation and running water such as the lower side of leaning trunks or cavities at the base of trees. The species is found on trees with acidic bark, in eastern Asia mainly on Betula but also on coniferous trees and on dead wood. It was not found on bark of Salix ( caprea ), a common substrate for the species in Europe.