King of Prussia is a census-designated place in Upper Merion Township, Montgomery County, Pennsylvania, United States. As of the 2010 census, its population was 19,936. The community took its name in the 18th century from a local tavern named the King of Prussia Inn, which was named after King Frederick II of Prussia. Like the rest of Montgomery County, King of Prussia continues to experience rapid development. The second-largest shopping mall in the United States in terms of space and size (nearly 3 million square feet), the King of Prussia Mall, is located here. Also located here is the headquarters of the Nuclear Regulatory Commission Region I.