The Fairfax Line was a surveyor's line run in 1746 to establish the limits of the "Northern Neck land grant" (also known as the "Fairfax Grant") in colonial Virginia.
The land grant, first contrived in 1649, encompassed all lands bounded by the Potomac and Rappahannock Rivers, an area of 5,282,000 acres (21,380 km2). By 1719, the lands had been inherited by Thomas Fairfax, 6th Lord Fairfax of Cameron (1693–1781). By that time the question of the boundaries of the designated lands had also become highly contentious. In 1745 it was decided that a line between the sources of the North Branch Potomac River and the Rappahannock River would constitute the western limit of Lord Fairfax's lands.