Ansonia is a city in New Haven County, Connecticut, United States. Located on the Naugatuck River, it is immediately north of Derby, and about 12 miles northwest of New Haven. The city is part of the Naugatuck Valley Planning Region. The population was 18,918 at the time of the 2020 census. The ZIP code for Ansonia is 06401. The city is served by the Metro-North Railroad.