Greenbriar is a census-designated place in Fairfax County, Virginia, United States. The population at the 2020 census was 8,421. The community between Fairfax City and Chantilly dates from the late 1960s, when it was developed by Levitt& Sons. The community is famous for having only one road within its territory that does not start with an "M" or "P", which is Acorn Hill. A popular belief is that the roads were named after the architect's two children.