Arlington County is an urban county of about 203,000 residents in the Commonwealth of Virginia, in the U.S., directly across the Potomac River from Washington, D.C. Formerly part of the District of Columbia, the land now comprising the county was retroceded to Virginia in a July 9, 1846, act of Congress that took effect in 1847.

Despite being a county, it is considered a Central City of the Washington, D.C. area by the Census, along with neighboring Washington and Alexandria. It is usually rendered "Arlington" or "Arlington, Virginia" by people and the media. At a land area of , it is geographically the smallest self-governing county in the United States.

CNN Money ranked Arlington as the most educated city in 2006. It is one of six Northern Virginia counties among the 20 highest median household income counties of the 2006 Census as reported by Forbes. The County is the location of Arlington National Cemetery, National Airport and the Pentagon.