Batillaria attramentaria, common name the Japanese mud snail, is a marine gastropod mollusk in the family Batillariidae. It is a species of sea snail most often found in the salt marshes and mudflats of marine, estuarine, riparian and wetland habitats. Introduced to North America between the 1920s to 1930s via the coasts of Washington and California, the Japanese mud snail became an invasive species notorious for reducing biodiversity by outcompeting the native hornsnail Cerithidea californica.