Nogales is a city in Santa Cruz County, Arizona, United States. The population was 21,017 at the 2010 census. The city is the county seat of Santa Cruz County. Nogales, Arizona, borders the city of Nogales, Sonora, Mexico, and is Arizona's largest international border town. The southern terminus of Interstate 19 is located in Nogales at the U.S. -Mexico border; the highway continues south into Mexico as Mexico Federal Highway 15.