Maricopa, AZ. Online Offers. Maricopa is a city in the Gila River Valley in the U.S. state of Arizona. Incorporated in 2003, it is the nineteenth largest city in the state with a population of 43,482 as of April 2010. Maricopa is the second most populous incorporated city in Pinal County and forms part of the Phoenix metropolitan area, the 12th largest metropolitan area in the United States. Maricopa has had three locations over the years: Maricopa Wells, Maricopaville and Maricopa Junction which gradually became known as Maricopa.