Bethesda is an unincorporated, census-designated place in southern Montgomery County, Maryland. It is located just northwest of Washington, D.C. It takes its name from a local church, the Bethesda Meeting House (1820, rebuilt 1849), which in turn took its name from Jerusalem's Pool of Bethesda. The National Institutes of Health's main campus and the Walter Reed National Military Medical Center are in Bethesda, in addition to a number of corporate and government headquarters.

As an unincorporated community, Bethesda has no official boundaries. According to the 2020 U.S. census, the community had a total population of 68,056.

According to the U.S. Census Bureau, the CDP has a total area of 13.2 square miles (34 km2), of which 13.1 square miles (34 km2) is land and 0.1 square miles (0.26 km2) (0.38%) is water. The main commercial corridor that passes through Bethesda is Maryland Route 355 (known as Wisconsin Avenue in Bethesda and as Rockville Pike and Hungerford Drive in more northern communities), which, to the north, connects Bethesda with the communities of North Bethesda and Rockville, ending, after several name changes, in Frederick. Toward the South, Rockville Pike becomes Wisconsin Avenue near the NIH Campus and continues beyond Bethesda through Chevy Chase, Friendship Heights and into Washington, D.C., ending in Georgetown.

The area commonly known as Downtown Bethesda is centered at the intersection of Wisconsin Avenue, Old Georgetown Road and East-West Highway. This intersection is approximately two and one-half miles from Washington, DC's western boundary, making Bethesda a close-in suburb of Washington. Other focal points of downtown Bethesda include the Woodmont Triangle, bordered by Old Georgetown Road (Maryland Route 187), Woodmont and Rugby Avenues, and the Bethesda Row, centered at the intersection of Woodmont Avenue and Bethesda Avenue. Much of the dense construction in that area followed the opening of the Bethesda station on the Red Line of the Washington Metro rapid transit system, also located at this intersection and the centerpiece of the Bethesda Metro Center development. The Medical Center Metro stop lies approximately 0.7 miles north of the Bethesda stop, Medical Center, which serves the NIH Campus, the Walter Reed National Military Medical Center, and the Uniformed Services University of the Health Sciences.