Xenostrobus securis Lamark 1819 is an elongate, blue black to brown colored mussel native to the southern coasts of Australia and to New Zealand. Endemic to brackish waters in its native range, it can tolerate a wide range of salinities and moderate to high temperatures, and is a successful invader of parts of China, Japan and Korea in the western Pacific Ocean, as well as in Italian, French and Spanish lagoons, rivers and coasts in the Mediterranean and eastern Atlantic. In December 2024 during surveys for the golden mussel Limnoperna fortunei, Xenostrobus securis was found by California Department of Fish and Wildlife in Dominguez Channel in Wilmington CA, part of the Los Angeles/Long Beach port complex, and verified by DNA sequencing. This is the first report of Xenostrobus securis on a United States coast. Likely vectors for introduction in California are hull fouling and/or ballast water. Xenostrubus securis can reproduce rapidly, colonize a variety of soft and hard substrates and overtake habitats, making it a highly successful colonizer and ecosystem engineer. It can also displace ecologically and economically important native species as well as become a fouling pest.