Maricopa is a city in Pinal County, Arizona, United States, thirty miles south of Phoenix in the Gila River Valley. As of 2008, its population is estimated to be 45,571, an increase of 44,531 over the 2000 census figure of 1,040. This makes it the largest city in the county and Arizona's seventeenth-largest city. Maricopa's annexation plans during the next decade are likely to result in major growth in both population and area .