The aquarium trade poses a great risk of aquatic invasion worldwide, but little attention has been paid to unintentionally introduced species. The present study reports the occurrence of the freshwater polychaete Namalycastis hawaiiensis for the first time from a private home aquarium in Japan, which was a considerable distance away from the native distribution of the species (tropical/subtropical regions). We concluded that the species was unintentionally introduced via the aquarium trade. As the specimens were collected from an aquarium of the common goldfish, which is one of the most popular ornamental fish in the world, it is possible that a cryptic invasion has already expanded worldwide.