Prince George’s County, also known as P.G. County, is a county in the U.S. state of Maryland, bordering the eastern portion of Washington, D.C. As of the 2010 U.S. Census, the population was 863,420, making it the second-most populous county in Maryland, behind only Montgomery County. Its county seat is Upper Marlboro. It is one of the richest African American-majority counties in the United States.