Cabin John is a census-designated place and unincorporated area in Montgomery County, Maryland, United States.

Cabin John is the location of the Union Arch Bridge, built between 1857 and 1864 over Cabin John Creek; 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.

Originally a refreshment stand and boarding house for bridge workers, it grew into "a sumptuous establishment frequented by the most powerful politicians and important social figures of Washington, D.C." In its heydays of the 1890s to early 1900s, "the hotel was so lavish and became so important a destination for Washingtonians that The Washington Post reported about it regularly."

[5]: 12 Bordering Cabin John to the west is Carderock, where in 1936–1939 the U.S. Navy built the David Taylor Model Basin, one of the largest test facilities for ship design in the world.

The test basin facility was originally planned for Cabin John, but instead the original 19.27-acre (7.80 ha) site, bought from Mary Ellen Bobinger, widow of the owner of the Cabin John Hotel, was used to build housing for workmen.

The Cabin John Gardens homes were constructed on the site of the former hotel and sold by the government to employees, with the land itself held as a cooperative—the only single-family development of its kind in the county.

Carver Road homes were located about 1/3 of a mile to the northwest and originally rented out for $28 a month.

"[4] Before the passage of the Fair Housing Act of 1968, racial covenants were used to exclude African Americans and other people of color.

[1] Before the construction of the Cabin John Parkway in the mid-1960s, the area was considered to extend north to River Road (Maryland Route 190).

[4] Historically, Cabin John has been part of the region known as the Potomac Gorge (Potomac Palisades), an approximately 15-mile-long (24 km) corridor or "fall zone" that sees a transition between the hard bedrock of the Piedmont to the softer rocks and soils of the Atlantic Coastal Plain and representing a drop in elevation from 140 feet (43 m) to about 10 feet (3.0 m) above sea level.