Fort Independence is a granite bastion fort that provided harbor defenses for Boston, Massachusetts. Located on Castle Island, Fort Independence is one of the oldest continuously fortified sites of English origin in the United States. The first primitive fortification, called "The Castle", was placed on the site in 1634 and, after two re-buildings, replaced circa 1692 with a more substantial structure known as Castle William. Re-built after it was abandoned by the British during the American Revolution, Castle William was renamed Fort Adams and then Fort Independence. The existing granite fort was constructed between 1833 and 1851. Today it is preserved as a state park and fires occasional ceremonial salutes. Fort Independence was added to the National Register of Historic Places in 1970.

The site of Fort Independence has been occupied by various fortifications since 1634. The first fort to be constructed on Castle Island resulted from a visit by Governor John Winthrop; it was partly financed by him and the council. Construction was planned and supervised by Deputy Gov. Roger Ludlow and Captain John Mason of Dorchester, producing a "castle with mud walls" with masonry of oyster shell lime, in which cannon were mounted to defend the colonial town of Boston from attack by sea. The first commander of the fort was Captain Nicholas Simpkins in 1634. The first fort soon fell into disrepair and was rebuilt, mainly out of timber, in 1644 following a scare due to the arrival of a French warship in the harbor. The fort was reconstructed out of pine logs, stone, and earth, with 10-foot walls around a compound 50 feet square. The fort mounted six saker cannons and three smaller guns. A later commander of the fort was Captain Richard Davenport, who supervised the post from 1645 until 1665 when he was struck by lightning within the fort and killed. His successor, Captain Roger Clap, commanded the fort from 1665–1686.