Dorcus koreanus Jang and Kawai, 2008 was recently described as a valid species in Haenam, South Korea, based on morphology. However, the taxonomic position and relationships of this new species with the related species Dorcus japonicus and Dorcus carinulatus were not examined in detail. To address this issue, we evaluated the phylogenetic relationships of D. koreanus to its related species based on molecular analyses of mitochondrial 16S rRNA gene sequences. The molecular evidence suggested that D. koreanus and D. carinulatus are more closely related to each other than either is to D. Japonicus. The genetic divergence between D. koreanus and D. carinulatus ranged from 1.2 to 1.6%, whereas that between D. koreanus and D. japonicus ranged from 9.0 to 9.2%. By comparing the range of nucleotide substitutions within Lucanidae, we determined that the sequence distance between D. koreanus and D. carinulatus is smaller than that for required for species-level distinction. Therefore, we reduce D. koreanus to subspecies rank, as Dorcus carinulatus koreanus stat. nov.