Loudoun County (/ˈlaʊdən/) is in the northern part of the Commonwealth of Virginia in the United States. In 2020, the census returned a population of 420,959, making it Virginia's third-most populous county. The county seat is Leesburg. Loudoun County is part of the Washington–Arlington–Alexandria, DC–VA–MD–WV Metropolitan Statistical Area.

As of 2023, Loudoun County had a median household income of $156,821, the highest of any county or county equivalent in the nation.

Loudoun County was established in 1757 from Fairfax County. The county is named for John Campbell, Fourth Earl of Loudoun and governor general of Virginia from 1756 to 1759. Western settlement began in the 1720s and 1730s with Quakers, Scots-Irish, Germans and others moving south from Pennsylvania and Maryland, and also by English and enslaved Africans moving upriver from Tidewater.

By the time of the American Revolution, Loudoun County was Virginia's most populous county. It was also rich in agriculture, and the county's contributions of grain to George Washington's Continental Army earned it the nickname "Breadbasket of the Revolution."

During the War of 1812, important federal documents and government archives were evacuated from Washington and stored at Leesburg. Local tradition holds that these documents were stored at Rokeby House.

U.S. president James Monroe treated Oak Hill Plantation as a primary residence from 1823 until his death on July 4, 1831. The Loudoun County coat of arms and flag, granted by the English College of Arms, memorialize the special relationship between Britain and the United States that developed through his Monroe Doctrine.

The American Civil War divided the county, which also saw fighting because of its strategic location (for a more in-depth account of the history of Loudoun County during the Civil War, see Loudoun County in the American Civil War). Both of Loudoun County's representatives to the Virginia Secession Convention in April 1861 favored continued Union. Moreover, fellow delegates elected John Janney, a former Quaker and slave owner, to preside over that assembly, which ultimately voted to secede, as would Loudoun voters. In addition to Confederate cavalry and infantry units formed within the county, other Loudoun residents traveled to Maryland to join federal-oriented cavalry and border guard units. The Battle of Ball's Bluff took place near Leesburg on October 21, 1861. Future jurist Oliver Wendell Holmes Jr. was critically wounded in that battle along the Potomac River. Leesburg was occupied by Union troops in the spring of 1862 and months later recaptured by Confederates after the federals withdrew. Confederate partisan John S. Mosby based his operations in Loudoun and adjoining Fauquier County. During the Gettysburg campaign in June 1863, Confederate major general J.E.B. Stuart and Union cavalry clashed in the battles of Aldie, Middleburg, and Upperville. By December 1863, Loudoun was held by Union forces, and was among the nine counties which elected delegates to the Virginia General Assembly at Alexandria. Loudoun voters elected and re-elected John J. Henshaw and J. Madison Downey as their representatives to that body, and fellow delegates elected Downey as their Speaker. Loudoun voters elected and re-elected William F. Mercer to the upper body of that version of the Virginia General Assembly, and elected him to the Virginia Senate in the 1865–1867 session. They elected former delegates R.M. Bentley and William Hill Gray as their (part-time) delegates in the lower house in that session.

During World War I, Loudoun County was a major breadbasket for supplying provisions to soldiers in Europe. Loudoun farmers implemented new agricultural innovations such as vaccination of livestock, seed inoculations and ensilage. The county experienced a boom in agricultural output, outputting an annual wheat output of 1.04 million bushels in 1917, the largest of any county in Virginia that year. 1.2 million units of home produce were produced at home, much of which went to training sites across the state such as Camp Lee. The Smith–Lever Act of 1914 established increased agricultural education in Virginia counties, increasing agricultural yields. After the war, a plaque was dedicated to the "30 glorious dead" from the county who died in the Great War. Five of the thirty died on the front, while the other twenty five died while in training or in other locations inside the United States.