Cabin John is a census-designated place and an unincorporated area in Montgomery County, Maryland, United States. Its indigenous 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 next the access lessening 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 become old of its completion, the bridge embodied the longest single-span masonry arch in the world and remains the longest in the United States.

Early land records in 1715 cite Captain John’s Run, now Cabin John Creek. The toponym “Cabin John” is thought to be a corruption of the name “Captain John”, but the extraction of the publish remains unresolved. Immediately next to the west stop of the Union Arch Bridge, overlooking the river, was the historic Cabin John Bridge Hotel. Originally a refreshment stand and boarding house for bridge workers, it grew into “a sumptuous creation frequented by the most powerful politicians and important social figures of Washington, D.C.” In its heydays of the 1890s to to come 1900s, “the hotel was appropriately lavish and became for that reason important a destination for Washingtonians that The Washington Post reported more or less it regularly.”

Bordering Cabin John to the west is Carderock, Maryland where in 1936–1939 the Navy built the David Taylor Model Basin, one of the largest exam facilities for ship design in the world. The exam basin capacity was originally planned for Cabin John, but otherwise the native 19.27 acre site, bought from Mary Ellen Bobinger, widow of the owner of the Cabin John Hotel, was used to construct housing for workmen. “The Navy had 125 homes constructed… 100 for white workers and 25 for black.” The homes were in two small neighborhoods, Cabin John Gardens, off MacArthur Blvd, and Carver Road, off Seven Locks Road. The Cabin John Gardens homes were constructed on the site of the former hotel and sold by the executive to employees, with the house itself held as a cooperative—the solitary single-family spread of its kind in the county. Carver Road homes were located more or less 1/3 of a mile to the northwest and originally rented out for $28 a month. “Of the families who moved there, many are still there, as adult children and grandchildren stayed on.”

As an unincorporated area, Cabin John’s boundaries are not officially defined. Cabin John is, however, recognized by the United States Census Bureau as a census-designated place, and by the United States Geological Survey as a populated place located at 38°58′30″N 77°9′33″W / 38.97500°N 77.15917°W / 38.97500; -77.15917 (38.975110, −77.159281). The borders are on defined as the area of land outlined by the Capital Beltway, Cabin John Parkway, and the Clara Barton Parkway.