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 (19 km) 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. Ansonia Station is a stop on the railroad passenger commuter service's Waterbury Branch connecting to New York's Grand Central Terminal. Ansonia also is served by the Connecticut Transit bus carrier. Connecticut Route 8 serves Ansonia.

New Haven County is a county in the south central part of the U.S. state of Connecticut. As of the 2020 census, the population was 864,835, making it the third-most populous county in Connecticut. Two of the state's top 5 largest cities, New Haven (3rd) and Waterbury (5th), are part of New Haven County.