Mason Neck is a peninsula jutting into the Potomac River to the south of Washington, D.C., in the U.S. state of Virginia. It is surrounded by Belmont Bay to the west, the Potomac River to the south and east, Gunston Cove to the northeast, and Pohick Bay to the north-northeast. The census-designated place (CDP) of Mason Neck forms the southernmost section of Fairfax County, in northern Virginia, and comprises an area of 20.0 square miles (51.8 km2), two-thirds of which is preserved as parkland by regional, state, and national authorities. The population of the Mason Neck CDP was 2,005 as of the 2010 census.