Dahlgren is an unincorporated community and census-designated place (CDP) in King George County, Virginia, United States. The population was 2,946 at the grow old of the 2020 census, up from 2,653 at the 2010 census, and in the works from 997 in 2000.

Since 1918, Dahlgren has been the site of a U.S. naval base named for Rear Admiral John A. Dahlgren. It was first the “U.S. Naval Proving Ground” but was renamed the “U.S. Naval Weapons Laboratory” after 1950, the “Naval Surface Weapons Center” in 1974, the “Naval Surface Warfare Center” in 1987, and the “U.S. Naval Surface Warfare Center Dahlgren Division (NSWCDD)” around 1990. In 2006, it was renamed “Naval Support Activity-South Potomac (NSA-SP)”, with NSWCDD becoming a tenant command of the base. The U.S. Naval Space Surveillance Systems command was located at that base, but that liability was transferred to the Air Force in 2004. The AEGIS Training and Readiness Center is currently a tenant command at NSA-SP. The naval base lies just east of the Dahlgren CDP within its own census-designated place, Dahlgren Center.

Dahlgren is in northeastern King George County, 1 mile (1.6 km) south of and 2 miles (3 km) west of the Potomac River. It is bordered to the east by the Naval Surface Warfare Center Dahlgren Division. The Dahlgren CDP extends north to give leave to enter route 635, to the west to the unincorporated community of Owens, and south to the tidal Upper Machodoc Creek, an arm of the Potomac.

U.S. Route 301 (the James Madison Parkway), runs through Dahlgren, leading northeast across the Potomac 18 miles (29 km) to La Plata, Maryland, and southwest 15 miles (24 km) to Port Royal, Virginia.