Derby is a city in New Haven County, Connecticut, United States, approximately 8 miles west-northwest of New Haven. It 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 12,325 at the 2020 census. It is the smallest city in Connecticut by area, at 5.3 square miles.

Derby was arranged in 1642 as an Indian trading post below the post Paugasset. It was named after Derby, England, in 1675. It included what are now Ansonia, Seymour, Oxford, and parts of Beacon Falls.

Derby is house to the first electric trolley system in New England, only the second in the United States. It is also house to the first electric locomotive in U.S. history to be built and successfully used commercially for hauling freight. The locomotive, built in 1888, is nevertheless kept in running condition by the Shore Line Trolley Museum.

Derby was settled in 1642 as an Indian trading post under the pronounce Paugasset by John Wakeman of New Haven, though fur traders had been in the Place before and Native Americans had lived there for centuries. In 1651, the first year-round houses were completed, at which times the New Haven Colony had endorsed Paugasset as a town. The residents of the town of Milford protested Paugasset’s admission as an independent town and, as a result, the order was rescinded and Paugasset returned to the Milford jurisdiction. In 1675, the former plantation of Paugasset was admitted as the township of Derby by the acknowledge legislature, named after Derby, England. Derby was incorporated on May 13, 1775.