Maricopa County (/ˌmærɨˈkoʊpə/ MARR-i-KOH-pə) is a county located in the south-central part of the U.S. state of Arizona. As of the 2010 census, its population was 3,817,117, making it the most populous county in the state, and the fourth-most populous in the United States. It is more populous than 23 states.