Arlington County is a county in the Commonwealth of Virginia, often referred to simply as Arlington or Arlington, Virginia. The county is situated in Northern Virginia on the southwestern bank of the Potomac River directly across from the District of Columbia, of which it was once a part, under the name Alexandria County. The county is coextensive with the U.S. Census Bureau‘s census-designated place of Arlington. Arlington County is considered to be the second-largest “principal city” of the Washington metropolitan area, although Arlington County does not have the legal designation of independent city or incorporated town under Virginia state law.
In 2019, the county’s population was estimated at 236,842, making Arlington the sixth-largest county in Virginia by population; if it were incorporated as a city, Arlington would be the fourth most populous city in the state. With a land area of 26 square miles (67 km2), Arlington is the geographically smallest self-governing county in the U.S., and by reason of state law regarding population density, it has no incorporated towns within its borders. It is the 5th highest-income county in the U.S. by median family income, and is the 11th most densely populated county in the United States.