Cape Cod, often referred to as simply the Cape, and called Cape of Keel by early Norse explorers, is a peninsula in the easternmost portion of the state of Massachusetts, in the Northeastern United States. It is coextensive with Barnstable County. Several smaller islands off Cape Cod, including Monomoy Island, Monomoscoy Island, Popponesset Island, and Seconsett Island, are also in Barnstable County, being part of municipalities with land on the Cape. The Cape's small-town character and large beachfront attract heavy tourism during the summer months.

Cape Cod was formed as the terminal moraine of a glacier, resulting in a peninsula in the Atlantic Ocean. In 1914, the Cape Cod Canal was cut through the base or isthmus of the peninsula. It is still identified as a peninsula by geographers, who do not change landform designations based on man-made canal construction.

Unofficially, it is one of the biggest barrier islands in the world, shielding much of the Massachusetts coastline from North Atlantic storm waves. This protection helps to erode the Cape shoreline at the expense of cliffs, while protecting towns from Fairhaven to Marshfield