Triangle is a census-designated place (CDP) in Prince William County, Virginia, United States. The population was 5,500 at the 2000 census. It is bounded to the south by the Quantico Marine Corps Base, which surrounds the town of Quantico. 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.