Dennis is a town in Barnstable County, Massachusetts, United States, near the center of the Cape Cod peninsula. It is a seaside resort town with colonial mansions along the northern Cape Cod Bay coastline and beaches along the southern Nantucket Sound. As of the 2020 census, its population was 14,674. The town encompasses five villages, each with its own post office: Dennis, Dennis Port, East Dennis, South Dennis, and West Dennis.