Arlington County is a county in the U.S. state of Virginia. The county is located in Northern Virginia on the southwestern bank of the Potomac River directly across from Washington, D.C.. The county is coextensive with the U.S. Census Bureau's census-designated place of Arlington. Arlington County is the second-largest city in the Washington metropolitan area, although it does not have the legal designation of an independent city or incorporated town under Virginia state law.

In 2020, the county's population was estimated at 238,643, making Arlington County the sixth-largest county in Virginia by population and the largest unincorporated community in the United States. If Arlington County were incorporated as a city, it would be the third-most populous city in Virginia. With a land area of 26 square miles (67 km2), Arlington is the geographically smallest self-governing county in the U.S. but has no incorporated towns under state law. It has the fifth-highest income per capita among all U.S. counties, and is the nation's 11th-most densely populated county.

Arlington is home to the Pentagon (headquarters of the U.S. Department of Defense), the Defense Advanced Research Projects Agency (DARPA), Reagan National Airport, and Arlington National Cemetery. In academia, the county contains Marymount University, the satellite campuses and research programs of George Mason University's Antonin Scalia Law School, Schar School of Policy and Government, and the Jimmy and Rosalynn Carter School for Peace and Conflict Resolution, as well as graduate programs, research, and non-traditional student education centers at the University of Virginia and Virginia Tech. Arlington is also the future home of the co-headquarters of Amazon, and the global headquarters of aerospace manufacturing and defense industry giants Boeing and Raytheon Technologies, as well as the American subsidiary of BAE Systems.