The Northern Neck Proprietary – also called the Northern Neck land grant, Fairfax Proprietary, or Fairfax Grant – was a land grant first contrived by the exiled English King Charles II in 1649 and encompassing all the lands bounded by the Potomac and Rappahannock Rivers in colonial Virginia. This constituted up to 5,000,000 acres (20,000 km2) of Virginia's Northern Neck and a vast area northwest of it.
The grant became actual in 1660 when Charles was restored to the English throne. By 1719, these 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. It was decided in 1746 that a line between the sources of the North Branch of the Potomac and the Rappahannock River (the "Fairfax Line") would constitute the western limit of Lord Fairfax's lands.
The unsettled portions of his domain were finally confiscated during the American Revolution by the Virginia Act of 1779 and when he died in 1781 the Proprietary effectively ceased to exist. A portion of this estate, however, was later the subject of the landmark Supreme Court case Martin v. Hunter's Lessee (1816).