Mesa is a city in Maricopa County, Arizona, United States. The population was 504,258 at the 2020 census. It is the third-most populous city in Arizona, after Phoenix and Tucson, the 38th-most populous city in the U.S., and the most populous city that is not a county seat. It is the most populous city in the East Valley of the Phoenix metropolitan area. It borders Tempe on the west, the Salt River Pima–Maricopa Indian Community on the north, Chandler and Gilbert on the south along with Queen Creek, and Apache Junction on the east.