Adult females of the ergasilid copepod Neoergasilus japonicus (Harada, 1930) were collected from the fins and skin of the smallmouth bass Micropterus dolomieu Lacepède, 1802, an invasive alien fish of North American origin, caught in a pond, Nagano Prefecture, central Japan. This is the first Japanese record of N. japonicus from M. dolomieu and the third species of parasite infecting this fish species in Japan.