Gainesville is a census-designated place (CDP) in Prince William County, Virginia, United States. The population was 11,481 in the 2010 census. According to the United States Census Bureau, the CDP has a total area of 10.3 square miles (26.6 km²), of which, 9.7 square miles (25.2 km²) of it is land and 0.6 square miles (1.5 km²) of it (5.45%) is water.
Gainesville was once a changing point for stagecoach horses on the Fauquier & Alexandria Turnpike. In 1852, the Manassas Gap Railroad reached the area and the stop became known as Gainesville. It was a shipping point for grain, timber, and cattle and remained a major cattle shipping point into the early 1960s. During theAmerican Civil War, nearby Thoroughfare Gap in the Bull Run Mountains served as a path for soldiers to reach the First and Second battles of Bull Run. In 1994, the groundbreaking for Gainesville's first townhome community began; it was named Crossroads. This marked the beginning of mass-development for Gainesville.