Map - Fairfax County, Virginia (Fairfax County)

Fairfax County, officially the County of Fairfax, is a county in the Commonwealth of Virginia. It is part of Northern Virginia and borders both the city of Alexandria and Arlington County and forms part of the suburban ring of Washington, D.C. The county is predominantly suburban in character with some urban and rural pockets.

As of the 2020 census, the population was 1,150,309, making it Virginia's most populous jurisdiction, with around 13% of the Commonwealth's population. The county is also the most populous jurisdiction in the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area, with around 20% of the MSA population, as well as the larger Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area, with around 13% of the CSA population. The county seat is Fairfax, although because it is an independent city under Virginia law, the city of Fairfax is not part of Fairfax County.

Fairfax was the first U.S. county to reach a six-figure median household income and has the third-highest median household income of any county-level local jurisdiction in the U.S. Fairfax County, as part of the Washington, D.C. metropolitan area, is usually included atop or near the top of lists of the wealthiest areas in the United States.

The county is home to the headquarters of several intelligence agencies such as the Central Intelligence Agency, National Geospatial-Intelligence Agency, National Reconnaissance Office, National Counterterrorism Center, and Office of the Director of National Intelligence as well as the headquarters of several aerospace manufacturing and defense industry giants. The county is also home to the flagship campus of George Mason University, and headquarters to seven Fortune 500 companies, including three in the Falls Church area, though not in the independent municipality of Falls Church.

At the time of first European encounter, the inhabitants of what would become Fairfax County were an Algonquian-speaking subgroup called the Taux, also known as the Doeg or Dogue. Their villages, as recorded by Captain John Smith in 1608, included Namassingakent and Nemaroughquand on the south bank of the Potomac River in what is now Fairfax County. Virginian colonists from the Northern Neck region drove the Doeg out of this area and into Maryland by 1670.

Fairfax County was formed in 1742 from the northern part of Prince William County. It was named after Thomas Fairfax, 6th Lord Fairfax of Cameron (1693–1781), proprietor of the Northern Neck. The Fairfax family name is derived from the Old English phrase for "blond hair", Fæger-feax.

The oldest settlements in Fairfax County were along the Potomac River. George Washington settled in Fairfax County and built his home, Mount Vernon, facing the river. Gunston Hall, the home of George Mason, is nearby. Modern Fort Belvoir is partly on the estate of Belvoir Manor, built along the Potomac by William Fairfax in 1741.

Thomas Fairfax, the only member of the British nobility ever to reside in the colonies, lived at Belvoir before moving to the Shenandoah Valley. The Belvoir mansion and several of its outbuildings were destroyed by fire immediately after the Revolutionary War in 1783, and George Washington noted the plantation complex deteriorated into ruins.

In 1757, the northwestern two-thirds of Fairfax County became Loudoun County. In 1789, part of Fairfax County was ceded to the federal government to form Alexandria County of the District of Columbia. Alexandria County was returned to Virginia in 1846, reduced in size by the secession of the independent city of Alexandria in 1870, and renamed Arlington County in 1920. The Fairfax County town of Falls Church became an independent city in 1948. The Fairfax County town of Fairfax became an independent city in 1961.

Located near Washington, D.C., Fairfax County was an important region in the Civil War. The Battle of Chantilly or Ox Hill, during the same campaign as the second Battle of Bull Run, was fought within the county; Bull Run is the border between Fairfax and Prince William Counties. Other areas of activity included Minor's Hill, Munson's Hill, and Upton's Hill, on the county's eastern border, overlooking Washington, D.C.