The non-native amphipod Grandidierella japonica Stephensen, 1938 is reported for the first time on the Atlantic coast of mainland Europe, specifically from Marennes-Oléron Bay, France. Likely vectors for this introduction include commercial shellfish transplants and ballast waters. A native of Japan, this species had previously only been reported twice outside the Pacific region; in both cases from coastal waters of England. Adults of both sexes, females carrying eggs, and juveniles were collected in Marennes-Oléron Bay, which suggests the species has become established.