Great Diamond Island is an island in Casco Bay, Maine, United States. It is part of the city of Portland, Maine. At 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. After the base was decommissioned, the bunkers and residences were left idle for over 30 years before being developed and sold to private citizens.