Great Diamond Island is an island in Casco Bay, Maine. It is part of the city of Portland, Maine. As of the 2000 census, the island had a year-round population of 77. The island is not accessible from the mainland by motor vehicle and has a limited network of roads. The primary modes of transportation are golf carts and bicycles. This fact has become a selling point for the island, marketing the area as "car free" and "kid friendly." The island was used as a military base starting in the late 19th century and continuing through World War II.