Phoenix is the capital and most populous city of the U.S. state of Arizona. With over 1.6 million residents at the 2020 census, Phoenix is the fifth-most populous city in the United States and the most populous state capital. The Phoenix metropolitan area, with an estimated 5.19 million residents, is the tenth-most populous metropolitan area in the U.S. and the most populous in the Mountain states and Southwest. Phoenix is the county seat of Maricopa County in the Salt River Valley and Arizona Sun Corridor and, with an area of 517.9 square miles (1,341 km²), is the largest city by area in Arizona and 11th-largest city by area in the United States.