Maricopa County, Arizona. Maricopa County (/ˌmærᵻˈkoʊpə/ MARR-i-KOH-pə) is a county 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 state's most populous county, and the fourth-most populous in the United States.