The Harbor Defenses of Portland was a United States Army Coast Artillery Corps harbor defense command. It coordinated the coast defenses of Portland, Maine, the mouth of the Kennebec River, and surrounding areas from 1895 to 1950, beginning with the Endicott program. These included both coast artillery forts and underwater minefields. The command originated circa 1895 as the Portland Artillery District, was renamed Coast Defenses of Portland in 1913, and again renamed Harbor Defenses of Portland in 1925.

Fort Popham is a Civil War-era coastal defense fortification at the mouth of the Kennebec River in Phippsburg, Maine. It is located in sight of the short-lived Popham Colony and, like the colony, named for George Popham, the colony's leader. The site is preserved as Fort Popham State Historic Site.