The distribution of the non-indigenous mytilid bivalve Xenostrobus securis (Lamarck, 1819), native to Australasia, was investigated through a survey of 325 sites along the Japanese coast of the Sea of Japan from 2006 to 2011. The species was found at 20 sites from Toyama Prefecture westwards (Toyama Port and Shin-minato Fishing Port in Toyama Prefecture; Kanazawa Port and the Daishoji River in Ishikawa Prefecture; the Inokuchi River, Lake Kugushi, and Lake Suigetsu in Fukui Prefecture; the Takeno River in Hyogo Prefecture; Lake Nakaumi in Tottori and Shimane Prefectures; and Dokai Bay and Hakata Bay in Fukuoka Prefecture), all of which were located in brackish-water areas such as lagoons, river mouths, and the innermost parts of large ports. Size distributions of the collected mussels suggest that breeding populations have been established at eight sites. Although X. securis was not found at 86 sites from Niigata Prefecture northwards, water temperature regimes in the native and invaded areas suggest that it may eventually spread as far north as the southern end of Hokkaido.