Cabin John is a census-designated place and an unincorporated area in Montgomery County, Maryland, United States. Its original southern boundary encompassed a section of the Chesapeake and Ohio Canal (C&O) including locks 8–12, and the northern shore of the Potomac River. It is also the access point to Plummers Island, originally owned by The Washington Biologists’ Field Club and called “the most thoroughly studied island in North America.” Cabin John is the location of the Union Arch Bridge built between 1857 and 1864; at the time of its completion, the bridge embodied the longest single-span masonry arch in the world and remains the longest in the United States.