The Great Dismal Swamp is a large swamp in the Coastal Plain Region of southeastern Virginia and northeastern North Carolina, between Norfolk, Virginia, and Elizabeth City, North Carolina. It is located in parts of the southern Virginia independent cities of Chesapeake and Suffolk and northern North Carolina counties of Gates, Pasquotank, and Camden. Some estimates place the original size of the swamp at over one million acres (4,000 km2). As of 2022 the size of the Great Dismal Swamp is around 750 square miles.

A swamp is a forested wetland. Swamps are considered to be transition zones because both land and water play a role in creating this environment. Swamps vary in size and are located all around the world. The water of a swamp may be fresh water, brackish water, or seawater. Freshwater swamps form along large rivers or lakes where they are critically dependent upon rainwater and seasonal flooding to maintain natural water level fluctuations. Saltwater swamps are found along tropical and subtropical coastlines. Some swamps have hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodic inundation or soil saturation. The two main types of swamp are "true" or swamp forests and "transitional" or shrub swamps. In the boreal regions of Canada, the word swamp is colloquially used for what is more formally termed a bog, fen, or muskeg. Some of the world's largest swamps are found along major rivers such as the Amazon, the Mississippi, and the Congo.