The Delmarva Peninsula is a large peninsula on the East Coast of the United States, occupied by most of Delaware and portions of Maryland and Virginia. The peninsula is 170 miles (274 km) long. In width, it ranges from 70 miles (113 km) to 12 miles (19 km) at the isthmus on its northern edge to less near its southern tip. It is bordered by the Chesapeake Bay on the west, the Delaware River, Delaware Bay, and Atlantic Ocean on the east, and the Elk River and its isthmus on the north.

The northern isthmus of the peninsula is transected by the sea-level Chesapeake and Delaware Canal. Several bridges cross the canal, and the Chesapeake Bay Bridge and the Chesapeake Bay Bridge-Tunnel join the peninsula to mainland Maryland and Virginia, respectively. Another point of access is Lewes, Delaware, reachable by ferry from Cape May, New Jersey.

Dover, Delaware's capital city, is the peninsula's largest city by population but the main commercial area is Salisbury, Maryland, near its center. Including all offshore islands (the largest of which is Kent Island in Maryland), the total land area south of the Chesapeake and Delaware Canal is 5,454 sq mi (14,130 km²). At the 2000 census the total population was 681,030, giving an average population density of 124.86 persons/sq mi (48.2 persons/km²).