Ajo is an unincorporated community in Pima County, Arizona, United States. It is the closest community to Organ Pipe Cactus National Monument. The population was 3,039 at the 2020 census. Ajo is located on State Route 85 just 43 miles (69 km) from the Mexican border.

Pima County is a county in the south central region of the U.S. state of Arizona. As of the 2020 census, the population was 1,043,433, making it Arizona's second-most populous county. The county seat is Tucson, where most of the population is centered. The county is named after the Pima Native Americans, also known as Tohono O'odham, who are indigenous to this area.