Cape Cod is a cape jutting out into the Atlantic Ocean in the easternmost portion of the state of Massachusetts, in the Northeastern United States. Its historic, maritime character and ample beaches attract heavy tourism during the summer months.

In 1914, the Cape Cod Canal was cut through the base or isthmus of the peninsula, turning nearly all of Cape Cod into what would technically be described as an island, though this term is not common in everyday speech.

Cape Cod stretches from Provincetown in the northeast to Woods Hole in the southwest. Cape Cod encompasses Barnstable County, and is in full jurisdiction of the Cape Cod Commission, which includes a small part of that area that is on the mainland side of the Cape Cod Canal (which includes Buzzards Bay, Bournedale and Sagamore Beach. Most, however, incorrectly see the canal as Cape Cod's western boundary. Historically, the entire Town of Bourne (which was a part of Sandwich until 1884) was considered Cape Cod hundreds of years before the canal was built. The Town of Sandwich also extends over the Cape Cod Canal onto the mainland side of Cape Cod.

Two road 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 limited passenger services onto the Cape. Cape territory is divided into fifteen towns with many villages; Provincetown lies at the tip of the peninsula.
The name "Cape Cod", as it was first used in 1602, applied only to the very tip of the peninsula. It remained that way for 125 years, until the "Precinct of Cape Cod" was incorporated as the Town of Provincetown. No longer in "official" use over the ensuing decades, the name came to mean all of the land east of the Manomet and Scusset rivers – essentially along the line that became the Cape Cod Canal. Separating the vast majority of the peninsula from the mainland, the creation of the canal turned the resultant landmass into an island. Indeed, most agencies, including the Cape Cod Commission and the Federal Emergency Management Agency (FEMA), treat the Cape as an island with regards to disaster preparedness, groundwater management, and the like. Most "Cape Codders" – residents of "The Cape" – refer to all land on the mainland side of the canal as "off-Cape." However, the legal delineation of Cape Cod, coincident to the boundaries of Barnstable County, extends to the northwest boundaries of the towns of Bourne and Sandwich – and small portions of each are located west of the canal.

Cape Cod Bay lies in between Cape Cod and the mainland – bounded on the north by a horizontal line between Provincetown and Marshfield. North of Cape Cod Bay (and Provincetown) is Massachusetts Bay, which contains the Stellwagen Bank National Marine Sanctuary, located 5 miles (8 km) north of Provincetown. The Atlantic Ocean is to the east of Cape Cod, and to the southwest of the Cape is Buzzards Bay. The Cape Cod Canal, completed in 1916, connects Buzzards Bay to Cape Cod Bay; its creation shortened the trade route between New York and Boston by 62 miles (100 km).
The elevation of Cape Cod ranges from 306 feet (93 m) at its highest point, at the top of Pine Hill, in the Bourne portion of Joint Base Cape Cod, down to sea level.
One of the biggest barrier islands in the world, Cape Cod shields much of the Massachusetts coastline from North Atlantic storm waves. This protection erodes the Cape's shoreline at the expense of its cliffs, while protecting towns from Fairhaven to Marshfield.

Cape Cod and the Islands are part of a continuous archipelagic region consisting of a thin line of islands stretching west to include Long Island, in New York. This region is historically and collectively known by naturalists as the Outer Lands.