Derby is a city in New Haven County, Connecticut, United States, approximately 8 miles (13 km) west-northwest of New Haven. It is located in southwest Connecticut at the confluence of the Housatonic and Naugatuck rivers. It shares borders with the cities of Ansonia to the north and Shelton to the southwest, and the towns of Orange to the south, Seymour to the northwest, and Woodbridge to the east. The city is part of the Naugatuck Valley Planning Region. The population was 12,325 at the 2020 census. It is the smallest city in Connecticut by area, at 5.3 square miles (14 km2).

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.