Korean (hard-shelled) mussels (Mytilus coruscus) are an economically important endemic marine bivalve mollusk of Korea; yet, the population has rapidly declined because of overharvesting and habitat competition from the invasive Mytilus galloprovincialis species. The population structures of M. coruscus and M. galloprovincialis were analyzed by next-generation sequencing using 5 microsatellite markers specifically developed for M. coruscus. M. galloprovincialis had an average of 5.4 alleles per locus (range = 2-10), with an average allelic richness of 4.9 per locus (range = 2.0-9.3). M. coruscus had an average of 5.7 alleles per locus (range = 2-13), with an average allelic richness of 5.2 per locus (range = 2.0-11.9). Excessive homozygosity was observed at 3 loci, which was assumed to be due to the presence of null alleles at these loci. Pairwise multilocus FST estimates showed that the M. coruscus and M. galloprovincialis populations were clearly separated. Six populations of M. galloprovincialis from the western, eastern, and southern coast of Korea formed 2 separate clusters, indicating that more than 2 populations of M. galloprovincialis have been introduced to the Korean Peninsula. Hybrids between M. coruscus and M. galloprovincialis were not identified, probably because of genetic differences or different habitat preferences. Further genetic information is required to perform selective breeding, population management, and restoration of M. coruscus.