Wyoming /en-us-Wyoming. oggwaɪˈoʊmɪŋ/ is a state in the Western United States. The western two thirds of the state is mostly covered by the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High Plains. While the tenth largest U.S. state by area, Wyoming is the least populous, with a U.S. Census estimated population of 544,270 in 2009. This is a 5.9% increase since 2000.