Derby ('dər-bē) is a city in New Haven County, Connecticut, United States, approximately 8 miles west-northwest of New Haven, Connecticut. Derby is located in southwest Connecticut at the confluence of the Housatonic and Naugatuck Rivers. It borders 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 population was 12,903 at the 2010 census. It is the smallest consolidated city in Connecticut by area and population.
Derby was settled in 1642 as an Indian trading post under the name Paugasset. It was named after Derby, England, in 1675. It included what is now Ansonia, Seymour, Oxford, and parts of Beacon Falls. Derby is home to the first trolley system in New England and the first electric locomotive in U.S. history to be built and successfully used commercially for hauling freight. The locomotive, built in 1888 is still kept in running condition by the Shore Line Trolley Museum.