Arlington County, or simply Arlington, is a county in the U.S. state of Virginia. The county, which is located in the Washington metropolitan area and the broader Northern Virginia region, is positioned directly across from Washington, D.C., the national capital, on the southwestern bank of the Potomac River. The smallest self-governing county in the United States by area, Arlington County has both suburban and urbanized districts. Its urbanized district are located in proximity to several Washington Metro stations and lines. Its seat of government is located in the Court House neighborhood, which hosts many of its administrative offices and county courthouse.

Originally part of the Nacotchtank tribe's territory prior to the establishment of the Colony of Virginia in 1606, English colonists began settling in Arlington by the 1670s; the area was eventually designated as part of Fairfax County in 1742. The colonial-era economy was mostly based in tobacco agriculture operated with enslaved labor and indentured servants on large plantations. Following the end of the Revolutionary War, Arlington's planters and yeoman farmers transitioned to other crops. Virginia ceded present-day Arlington to help form the District of Columbia, and from 1801 the area was known as Alexandria County; it was eventually retroceded back to Virginia in 1847 as a result of pressure from Alexandria, which was the county's primary commercial center.

During the Civil War, Arlington formed part of the Union's defenses of Washington, which devastated its landscape and economy. Virginia's Reconstruction era Constitution of 1870, in addition to administratively separating Arlington from Alexandria, empowered its black community to participate in local and state elections, which changed the political dynamics of the county until local conservative Southern Democrats succeeded in reestablishing white supremacy by the 1880s. This facilitated the institution of Jim Crow laws and racial segregation in Arlington by the early 20th century. Developers and local politicians further ingrained these practices in Arlington as it experienced a boom in suburbanization with its expanding interurban trolley network starting in the 1880s and a continued influx of federal employees into the 1950s. Despite opposition from Virginia's massive resistance campaign and other groups, Arlington became the first county in Virginia to desegregate its schools in 1959, and its businesses in 1960 after a series of sit-in protests.

Arlington's modern economic development has been greatly influenced by its Metrorail lines, which through deliberate planning starting in the 1960s have become the center of its urban corridors. Many federal government agencies and complexes, including the Pentagon, which houses the headquarters of the United States Department of Defense, are based in Arlington. Government contractors that serve these organizations and others in the Washington metropolitan area, such as Boeing and RTX Corporation, are also located in the county. While the public sector is a primary driver of Arlington's economy, tech firms with private sector operations, including Amazon, have established regional headquarters or offices in Arlington's business districts over the past several decades. Institutions of higher education with main or satellite campuses in the county include George Mason University, Marymount University, the University of Virginia, and Virginia Tech. Arlington is also known as the location of Arlington National Cemetery, a military cemetery established in 1864 where more than 400,000 members of the U.S. Armed Forces are buried. Sites at the cemetery such as the Tomb of the Unknown Soldier attract thousands of visitors annually. Other prominent military memorials located in Arlington include the Marine Corps War Memorial and Air Force Memorial.

Arlington County was inhabited by prehistoric Native American cultures from the arrival of the Paleo-Indians 10,000 years before European colonization. Archeological evidence, including pottery fragments, tools, and arrowheads, suggests sporadic habitation during the Archaic Period, with more permanent communities during the Formative stage and Early Woodland period. Some objects unearthed during archeological digs have been found to be from as far as southern Ontario, indicating the existence of a trade route that ran through the area.

When John Smith made contact in 1608, Arlington was populated by the Nacotchtank, an Eastern Algonquian-speaking people that were likely part of the Powhatan Confederacy. He identified a village named Nameroughquena near the present-day 14th Street bridges. The Nacotchtank farmed, hunted, and fished along the nearby Anacostia River, where they had more villages.

Later in the 17th century, the Nacotchtank became involved in the regional beaver trade instigated by Henry Fleete, which while enabling the Nacotchtank to build wealth, undermined traditional social structures and ultimately weakened them. Further pressures, including population loss due to the spread of infectious diseases from Europeans, warfare with encroaching British colonizers, and conflict with Native American tribes in northern regions forced the Nacotchtank to abandon their homeland. By 1679, they had fully left the area and were absorbed into the Piscataway.

Colonists began migrating from Jamestown and towards the Potomac River between 1646 and 1676, during which land speculation in the region increased substantially. Early grants were issued in the mid-17th century by the Governor on behalf of the Crown to prominent figures of Virginia society; many never inhabited their landholdings during this period. The first "seated" grant in the area was the Howson Patent, which John Alexander purchased from Captain Robert Howson on November 13, 1669, and populated with tenants by 1677. With the establishment of the Northern Neck Proprietary after the restoration of Charles II in 1660, future land grants in the region were made by inheritors of Northern Neck, namely the Lords Fairfax.