Casco Bay is an inlet of the Gulf of Maine on the southern coast of Maine, New England, United States. Its easternmost approach is Cape Small and its westernmost approach is Two Lights in Cape Elizabeth. The city of Portland sits along its southern edge and the Port of Portland lies within.
There are two theories on the origin of the name "Casco Bay". Aucocisco is the Abenaki name for the bay, which means 'place of herons' (sometimes translated as 'muddy'). The Portuguese explorer Estêvão Gomes, mapped the Maine coast in 1525 and named the bay "Bahía de Cascos" (Bay of Helmets, based on the shape of the bay).
The first settlement in Casco Bay was that of Capt. Christopher Levett, an English explorer, who built a house on House Island in 1623-24. The settlement failed. The first permanent settlement of the bay was named Casco; despite changing names throughout history, that settlement remains the largest city in the Casco Bay region, now called the city of Portland, Maine
It was first reported in 1700 by Colonel Wolfgang William Römer, an English military engineer, that there were "as many islands as there are days in the year". The United States Coastal Pilot lists 136 islands, leading to the bay's islands being called that Calendar Islands based on the popular myth there are 365 of them. Later, Robert M. York, the former Maine state historian said there are "little more than two hundred islands".
Casco Bay is also home to abandoned military fortifications dating from the War of 1812 through World War II; during World War II, Casco Bay served as an anchorage for US Navy ships.