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.