Colchester is a town in New London County, Connecticut, United States. The town is part of the Southeastern Connecticut Planning Region. The population was 15,555 at the 2020 census. In 2010 Colchester became the first town in Connecticut, and the 36th in the country, to be certified with the National Wildlife Federation as a Community Wildlife Habitat.

New London County is in the southeastern corner of Connecticut and comprises the Norwich-New London, Connecticut Metropolitan Statistical Area, which is also included in the Hartford-East Hartford, Connecticut Combined Statistical Area. There is no county government and no county seat, as is the case with all eight of Connecticut's counties; towns are responsible for all local government activities, including fire and rescue, snow removal, and schools.