Maricopa County is 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 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. 4K Resolution.