Maricopa County is a county in the south-central part of the U.S. state of Arizona. As of the 2020 census the population was 4,420,568, or about 62% of the state's total, making it the fourth-most populous county in the United States and the most populous county in Arizona, and making Arizona one of the nation's most centralized states. The county seat is Phoenix, the state capital and fifth-most populous city in the United States.