Fort McKinley, Maine, Military heritage site on Great Diamond Island, Maine, United States.

Fort McKinley is a former U.S. Army coastal defense fort on Great Diamond Island in Casco Bay, Maine. It covers 111 acres divided by Diamond Cove into a North Fork and a South Fork, with nine gun batteries completed by 1906. The site also includes barracks, officers' quarters, and support buildings, and the Fort McKinley Historic District was listed on the National Register of Historic Places in 1985.

Fort McKinley's land was bought in 1873, but construction only began in 1897 after the Endicott Board recommended new fortifications in 1885. The fort was complete by 1906 with nine gun batteries, and a major barracks expansion finished in 1910. During World War I its guns were partly disarmed for possible railway use, and by the end of 1943 nearly all armament had been scrapped. The site was transferred to the Navy after closure and sold to private owners in 1961.

The site is named after an American president and still shows traces of its past as a military outpost in coastal waters. Visitors can see how soldiers lived and worked in this remote position.

Fort McKinley occupies Great Diamond Island in Casco Bay, reached by ferry from Portland; there is no bridge. The site is private property, and the Fort McKinley Historic District was listed on the National Register in 1985. Fort Lyon stood on Cow Island just north, was closed in 1946 and handed to the City of Portland.

The fort was paired with a smaller military installation on a nearby island, which now functions as an environmental education center. This unexpected transformation shows how old military structures can serve new purposes.