Mount Rainier is a city in Prince George's County, Maryland, United States, bordering Washington D.C. The population was 8,333 at the 2020 census. Mount Rainier is contained between the Northwest Branch Anacostia River, Cedar Lane Alley, and 34th Street to the north, 37th Street and 37th Place to the northeast, Upshur Street and Queens Chapel Road to the west, the CSX Capital Subdivision train tracks to the east, and Eastern Avenue NE to the south.