|— Census-designated place —|
|Adelphi Mill in July 2007.|
|Country||United States of America|
|• Total||2.7 sq mi (7.1 km2)|
|• Land||2.7 sq mi (7.1 km2)|
|• Water||0.0 sq mi (0.0 km2)|
|Elevation||171 ft (52 m)|
|• Density||5,500/sq mi ( 2,100/km2)|
|Time zone||Eastern (EST) (UTC-5)|
|• Summer (DST)||EDT (UTC-4)|
|ZIP codes||20783, 20787|
|GNIS feature ID||0596997|
Adelphi is an unincorporated area and census-designated place in Prince George's County, Maryland, United States. The population was 15,086 at the 2010 census. It includes the communities of Adelphi Hills, Buck Lodge, Chatham, Cool Spring Terrace, and White Oak Manor.
Bordering areas 
- Hillandale (north)
- College Park (east)
- University Park (southeast)
- Hyattsville (southeast)
- Chillum (south)
- Langley Park (southwest)
- Silver Spring (west; in Montgomery County)
The unincorporated Adelphi community takes its name from the historic Adelphi Mill, established in 1796 along the Northwest Branch of the Anacostia River. The Adelphi Mill is administered by the Maryland-National Capital Park and Planning Commission and continues to attract visitors and can be rented for special functions. During the 19th century, George Washington Riggs acquired much of the area northeast of Washington, D.C., as his Green Hill estate in the Chillum Manor district. That estate included present-day Adelphi. In the early 1920s, part of the area was acquired by Leander McCormick-Goodhart as part of his Langley Park estate. Labor organizer Mary Harris "Mother" Jones died in 1930, at the farm of Walter and Lillie May Burgess near Powder Mill and Riggs Roads in present day Adelphi. A marker was erected by the Maryland State Highway Administration commemorating Mother Jones at her death site.
Unlike its nearby neighbors Hyattsville and College Park, the area remained relatively undeveloped until immediately following World War II. At that time, subdivisions quickly developed. To distinguish it from the other neighboring unincorporated West Hyattsville communities of Lewisdale and Langley Park, the growing subdivisions banded together in the mid-1950s and adopted the name "Adelphi" to reflect their commitment to the preservation of the historic mill of the same name. These subdivisions now include Adelphi Hills, Adelphi Knolls, Buck Lodge, Chatham, Cool Spring Terrace, Heitmuller Estates, Hillandale Forest, Knollwood, Riggs Hill Manor, and White Oak Manor. A community focal point originally developed in the late 1950s is the Adelphi Pool.
In 1971, the Harry Diamond Laboratories was established on 137 acres (0.55 km2) in the northern part of Adelphi. That facility continues as the Adelphi Laboratory Center, Army Research Laboratory (ARL), the U.S. Army's corporate research laboratory. Other adjacent federal facilities include the National Archives at College Park and headquarters of the Food and Drug Administration.
Historic sites 
The following is a list of historic sites in Adelphi identified by the Maryland-National Capital Park and Planning Commission:
|Site name||Image||Location||M-NCPPC Inventory Number||Comment|
|1||Adelphi Mill and Storehouse||8401 and 8402 Riggs Road||65-006|
|2||Cool Spring Farm (Miller’s House)||2201 Cool Spring Road||65-005|
|3||Gallant House||3124 Powder Mill Road||61-013|
Adelphi is located at .(38.996860, -76.966755)
As of the census of 2000, there were 14,998 people, 5,332 households, and 3,321 families residing in the CDP. The population density was 5,067.9 people per square mile (1,956.3/km²). There were 5,627 housing units at an average density of 1,901.4/sq mi (734.0/km²). The racial makeup of the CDP was 29.48% White, 39.83% African American, 0.28% Native American, 9.95% Asian, 0.06% Pacific Islander, 15.04% from other races, and 5.36% from two or more races. Hispanic or Latino of any race were 25.74% of the population.
There were 5,332 households out of which 30.6% had children under the age of 18 living with them, 42.5% were married couples living together, 13.5% had a female householder with no husband present, and 37.7% were non-families. 27.5% of all households were made up of individuals and 6.1% had someone living alone who was 65 years of age or older. The average household size was 2.77 and the average family size was 3.35.
In the CDP the population was spread out with 22.7% under the age of 18, 13.0% from 18 to 24, 35.7% from 25 to 44, 19.1% from 45 to 64, and 9.5% who were 65 years of age or older. The median age was 32 years. For every 100 females there were 98.1 males. For every 100 females age 18 and over, there were 96.6 males.
The median income for a household in the CDP was $45,827, and the median income for a family was $53,839. Males had a median income of $32,495 versus $31,932 for females. The per capita income for the CDP was $20,952. About 6.6% of families and 9.0% of the population were below the poverty line, including 6.2% of those under age 18 and 7.4% of those age 65 or over.
Government and infrastructure 
- Adelphi Elementary School
- Cherokee Lane Elementary School
- Cool Spring Elementary School
- Mary Harris "Mother" Jones Elementary School
- Buck Lodge Middle School
- High Point High School
Private and parochial schools in Adelphi include:
The Adelphi community is located adjacent to the Langley Park shopping center and within a few miles of the regional Mall at Prince Georges. Additional shopping opportunities are available along University Boulevard at the Adelphi Shopping Center and Riggs Road at Metzerott Plaza. Until 2010, Adelphi was home to the original Ledo Restaurant, an Italian restaurant with pizza that remains a favorite of University of Maryland and community residents.
- U.S. Geological Survey Geographic Names Information System: Adelphi, Maryland
- "Profile of General Population and Housing Characteristics: 2010 Demographic Profile Data (DP-1): Adelphi CDP, Maryland". U.S. Census Bureau, American Factfinder. Retrieved December 14, 2011.
- "Adelphi Mill Is Reopened to the Public," The Washington Post and Times Herald, Dec 6, 1954, pg. 21.
- "Adelphi Laboratory Center". Entry for Adelphi Laboratory Center [Harry Diamond Laboratories], US Army Research Laboratory (ARL). Globalsecurity.org. 2008-04-15.
- M-NCPPC Illustrated Inventory of Historic Sites (Prince George's County, Maryland), 2006.
- "US Gazetteer files: 2010, 2000, and 1990". United States Census Bureau. 2011-02-12. Retrieved 2011-04-23.
- "Geographic Identifiers: 2010 Demographic Profile Data (G001): Adelphi CDP, Maryland". U.S. Census Bureau, American Factfinder. Retrieved December 14, 2011.
- "American FactFinder". United States Census Bureau. Retrieved 2008-01-31.
- "Contact/Visit Us." University System of Maryland. Retrieved on September 18, 2012. "3300 Metzerott Road Adelphi, MD 20783" - See also Directions to USM Office
- Cool Spring Terrace Civic Association
- Adelphi Pool
- Chillum-Adelphi Volunteer Fire Department
- History of the Army Research Laboratory
- Mother Jones Plaque in Adelphi, MD
- A scalable PDF map of Prince George's County, MD, showing the boundaries of the Adelphi Census Designated Place and other CDPs and Census Incorporated Places in the county -- from the Maryland Department of Planning web site