Fort Belvoir ( /ˈbɛlvwɑr/) is a United States Army installation and a census-designated place (CDP) in Fairfax County, Virginia, United States. Originally, it was the site of the Belvoir plantation. Today, Fort Belvoir is home to a number of important United States military organizations. The population of Fort Belvoir was 7,100 at the 2010 census.

The Fort Belvoir site was originally the home of William Fairfax, the cousin and land agent of Thomas Fairfax, 6th Lord Fairfax of Cameron the proprietor of the Northern Neck, which once stood on land now on the base. William Fairfax purchased the property in 1738 when his cousin arranged for him to be appointed customs agent (tax collector) for the Potomac River, and William erected an elegant brick mansion overlooking the river, moving in with his family in 1740. Lord Fairfax came to America in 1747 and stayed less than year at the Belvoir estate before moving to Greenway Court. The Fairfax family lived at Belvoir for over 30 years, but eldest son (and heir) George William Fairfax sailed to England on business in 1773, never to return. The manor home was destroyed by fire in 1783. Today, the ruins of the Belvoir Mansion and the nearby Fairfax family grave site are listed on the National Register of Historic Places. William Fairfax served in the Royal Navy as a young man and all of his sons (except his eldest, George William) served in uniform, two of whom were killed in combat.