Adelphi is an unincorporated area and census-designated place in Prince George's County, Maryland, United States. Per the 2020 Census, the population was 16,823. Adelphi includes the following subdivisions; Adelphi, Adelphi Park, Adelphi Hills, Adelphi Terrace, Adelphi Village, Buck Lodge, Chatham, Cool Spring Terrace, Hillandale Forest, Holly Hill Manor, Knollwood, Lewisdale, and White Oak Manor.

Prince George's County is located in the U.S. state of Maryland bordering the eastern portion of Washington, D.C. As of the 2020 U.S. census, the population was 967,201, making it the second-most populous county in Maryland, behind neighboring Montgomery County. The 2020 census counted an increase of nearly 104,000 in the previous ten years. Its county seat is Upper Marlboro. It is the largest and the second most affluent African American-majority county in the United States, with five of its communities identified in a 2015 top ten list.