Darnestown is a United States census-designated place and an unincorporated area in Montgomery County, Maryland. The CDP is 17.70 square miles with the Potomac River as its southern border and the Muddy Branch as much of its eastern border. Seneca Creek borders portions of its north and west sides. The Travilah, North Potomac, and Germantown census-designated places are adjacent to it, as is the city of Gaithersburg. Land area for the CDP is 16.39 square miles.