Maricopa County (pronounced /ˌmærɨˈkoʊpə/) is located in the south-central part of the U.S. state of Arizona. As of July 2008, its population was 3,954,598, which ranks fourth among the nation's counties and is greater than the population of 24 states. The county seat is Phoenix, which is Arizona's largest city and capital. The center of population of Arizona is located in Maricopa County, in the town of Gilbert. It is by far Arizona's most populous county, encompassing well more than half of the state's residents.

The population explosion is evident in a 2007 Forbes study which ranked four of Maricopa County's municipalities in the top ten fastest-growing cities in the nation. Those included Buckeye as the 2nd-fastest-growing city, Surprise and Goodyear as 3rd and 4th, and Avondale as 9th. All four of these cities are located in the booming "West Valley" which is the area of Maricopa County to the west of the City of Phoenix.

There are five Indian reservations located in the county. The largest of these are the Salt River Pima Maricopa Indian Community (East of Scottsdale) and the Gila River Indian Community (South of Phoenix), both of which have casino gambling.