Chebeague Island is located in Casco Bay, an inlet of the Gulf of Maine. It was originally used as a fishing ground by Abenaki Native Americans. Also known as Great Chebeague Island, today it is a town in Cumberland County, Maine, United States. It is located 10 miles from Portland, Maine. Chebeague Island is the largest island in Casco Bay that is not connected to the mainland by a bridge. The largest island is Sebascodegan, or "Great Island," which is part of the Town of Harpswell and connected to the mainland by a bridge.Chebeague Island is one of the twelve major islands of the Calendar Islands, a term that originated in 1700 with the report by the English military engineer Wolfgang William Römer, who claimed there were "as many islands as there are days in the year." As a corrective, the Maine State Historian Robert M. York stated there are "little more than two hundred islands" in Casco Bay.