Buellia numerosa and B. subnumerosa are described as new from Japan. These two corticolous species share common morphological characters: whitish, smooth to rimose thallus; adnate apothecia; flat, epruinose disc; dispersa-type proper exciple; numerous ascospores per ascus; and Buellia-type ascospores. In addition, they share chemical characters: presence of fumarprotocetraric acid and atranorin. They are clearly distinguishable from previously-described Buellia species based on their ascospore numbers and metabolites, and from each other by the number of ascospores per ascus and characters of the asci. Buellia numerosa has 20–64 ascospores per ascus while B. subnumerosa produces 8–36. Also, brown overmature asci are always found in B. subnumerosa, but not in B. numerosa. A phylogenetic analysis based on the ITS region confirmed that they should be recognized as two different species.