Dukes County is a county located in the U.S. state of Massachusetts. As of the 2020 census, the population was 20,600, making it the second-least populous county in Massachusetts. Its county seat is Edgartown. Dukes County comprises the Vineyard Haven, MA Micropolitan Statistical Area. The county consists of the island of Martha's Vineyard, the Elizabeth Islands, the island of Nomans Land, and other associated islets.