The Isle of Portland ( ) is a limestone island, long by wide, in the English Channel. Portland is south of the resort of Weymouth, forming the southernmost point of the county of Dorset, in England, United Kingdom. Chesil Beach connects the island to the mainland, and the A354 road bridge connects it to Weymouth, which together form the borough of Weymouth and Portland. The population of the island is almost 13,000.

Portland is a central part of the Jurassic Coast, a World Heritage Site on the Dorset and east Devon coast, important for its geology and landforms. The name of the island is used for one of the British Sea Areas, and has been exported as the name of North American and Australian towns. Portland limestone is still quarried here, and is used in British architecture, including St Paul's Cathedral and Buckingham Palace.

The large, deep artificial harbour on Portland's northern shore was a Royal Navy base during World War I and World War II; the Navy and NATO trained in its waters until the 1990s. The harbour is a small civilian port and popular recreation area; the Weymouth and Portland National Sailing Academy will host the sailing events for the 2012 Olympic Games.