Non-kin infanticide is the killing of unrelated young by a conspecific adult, and occurs infrequently in some bird species. We observed a case of non-kin infanticide committed by a male Japanese Marsh Warbler (Locustella pryeri) in 2008. The sudden disappearance of a nesting male from his territory made the neighboring male expand his original territory to the vacant area where the unrelated nestlings were being reared by a female in the nest. The male found the nest and attacked the unrelated nestlings. We inferred that the infanticide sequence was done as food resource competition and/or sexually selected infanticide.