Triangle is a census-designated place (CDP) in Prince William County, Virginia, United States. 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. According to the United States Census Bureau, the CDP has a total area of 2.6 square miles (6.8 Square Kilometer), all of it land.