Sigma C protein-coding sequences have been used to phylogenetically classify avian reovirus (ARV) strains. However, the relationship between serotype and phylogenetic cluster classification of the five prototype serotype strains of ARV in Japan has not been established. Thus, we used sigma C protein-coding sequences to characterize avian reoviruses (ARVs) isolated from chickens with tendonitis in Japan together with the five prototype serotype strains of ARV in Japan. Phylogenetic analysis of ARVs based on the sigma C protein-coding sequences revealed that the five prototype serotype strains of ARV were each classified into different, independent clusters. Two field isolates (JP/Tottori/2016 and JP/Nagasaki/2017) that were isolated from chickens with arthritis/tenosynovitis were classified into different clusters. JP/Tottori/2016 was classified into cluster VI with the CS-108 strain, and JP/Nagasaki/2017 was classified into cluster I with strain TS-142. Serologically, JP/Tottori/2016 was well-neutralized by antisera against the CS-108 strain, whereas JP/Nagasaki/2017 cross-reacted with antisera against both the CS-108 and TS-142 strains. Embryo lethality test revealed that the two field isolates induced 80% and 67% embryo mortality, respectively, whereas the five prototype strains induced 0%–33% embryo mortality. Our findings will contribute to understanding the characteristics of ARV strains in Japan.