Cape Cod is a peninsula extending into the Atlantic Ocean from the southeastern corner of Massachusetts, in the northeastern United States. Its historic, maritime character and ample beaches attract heavy tourism during the summer months. The name Cape Cod, coined in 1602 by Bartholomew Gosnold, is the ninth-oldest English place-name in the U.S.

As defined by the Cape Cod Commission's enabling legislation, Cape Cod is coextensive with Barnstable County, Massachusetts. It extends from Provincetown in the northeast to Woods Hole in the southwest, and is bordered by Plymouth to the northwest. The Cape is divided into fifteen towns, several of which are in turn made up of multiple named villages. Cape Cod forms the southern boundary of the Gulf of Maine, which extends north-eastward to Nova Scotia.

Since 1914, most of Cape Cod has been separated from the mainland by the Cape Cod Canal. The canal cuts 7 miles (11 km) roughly across the base of the peninsula, though small portions of the Cape Cod towns of Bourne and Sandwich lie on the mainland side of the canal. Two highway bridges cross the Cape Cod Canal: the Sagamore Bridge and the Bourne Bridge. In addition, the Cape Cod Canal Railroad Bridge carries railway freight and provides limited passenger service onto the Cape.

Like Cape Cod itself, the islands south of the Cape have evolved from whaling and trading areas to become resort destinations, attracting wealthy families, celebrities, and tourists. These include the large nearby islands of Nantucket and Martha's Vineyard. Both islands are also famous summer tourist destinations, commonly accessed by ferry from several locations on the cape. The phrases Cape Cod and the Islands and the Cape and Islands are often used to describe the whole region of Barnstable County, Dukes County (including Martha's Vineyard and the smaller Elizabeth Islands), and Nantucket County.

Several small islands right off Cape Cod, including Monomoy Island, Monomoscoy Island, Popponesset Island, and Seconsett Island, are also in Barnstable County.

The Forbes family-owned Naushon Island was first purchased by John Murray Forbes. Naushon is one of the Elizabeth Islands, many of which are privately owned. One of the publicly accessible Elizabeths is the southernmost island in the chain, Cuttyhunk Island, with a year-round population of 52 people. Several prominent families have established compounds or estates on the larger islands, making these islands some of the wealthiest resorts in the Northeast, yet they retain much of the early merchant trading and whaling culture.

Cape Cod in particular is a popular retirement area; 31.8% of the population of Barnstable County is 65 years old or older., and the average age of residents is the highest of any area in New England. Cape Cod is majority Democrat, but by a smaller margin than the rest of Massachusetts.

The bulk of the land in the area is glacial terminal moraine and represents the southernmost extent of glacial coverage in southeast New England; similar glacial formations make up Long Island in New York and Block Island in Rhode Island.