Triangle is a census-designated place (CDP) in Prince William County, Virginia, United States. The population was 8,188 at the 2010 census. It is bounded to the south by the Marine Corps Base Quantico, which surrounds the town of Quantico. According to the United States Census Bureau, the CDP has a total area of 2.6 square miles (6.8 km²), all of it land. It is bounded to the north and west by the town of Dumfries. It is bounded to the west by Prince William Forest Park and to the east by the Potomac River.