Fronteras is the seat of Fronteras Municipality in the northeastern part of the Mexican state of Sonora. Frontera translates as Border. The elevation is 1,120 meters and neighboring municipalities are Agua Prieta, Nacozari and Bacoachi. The area is 2839.62 km², which represents 1.53% of the state total.
Fronteras is located in a mountainous area on the west side of the Sierra Madre Occidental. The average annual temperature is 16.9°C. The rainy season is from July to August and the average annual rainfall is 427.5 millimeters.
The municipal population was 7,081 inhabitants (2.34 inhab/km²) in 2000, although in a second counting in 2005 this number had increased to 7,470 inhabitants. The most important settlement and the municipal seat had 874 inhabitants in 2000.
Industry is the most important economic activity together with agriculture and cattle raising. There was one maquiladora in 2000. The main agricultural crops were wheat, beans, corn, and grasses for cattle fodder. The cattle herd was substantial with over 30,000 head counted in 2000. Almost all were for meat production.
The Spanish explorer Juan Bautista de Anza (1735-1788), born in Fronteras, led the first Spanish overland expedition to the Las Californias Province of New Spain in 1769, previously only sighted and claimed for the Crown from the sea. He established the first Spanish settlement in present-day California, the Presidio of San Diego, and was the first European to see the San Francisco Bay.
Visitors can explore the Misión de Cuquiárachi founded in 1645 by the Jesuit missionary Marcos del Río. It is one of the Spanish missions in the Sonoran Desert. Two archaeological zones, of ancient Pima Indigenous people of the Americas settlement sites, are located near the village of Ojo de Agua. Other points of interest are the Jacinto López reservoir, and the Cave of Presidio de Fronteras.