county, southwestern New Mexico, U.S. Sierra county is in the Mexican Highland section of the Basin and Range Province. Its irregular western border is the Black Range, including Hillsboro and Reeds peaks, both rising to more than 10,000 feet (3,000 m). The Rio Grande, including large impoundments at Caballo and Elephant Butte reservoirs, flows southward through the centre of the county. Immediately east of the reservoirs are the Sierra Caballo and Fra Cristobal Range; the Jornada del Muerto desert separates those mountains from the San Andreas Mountains in the northeast. Sierra county includes Gila National Forest, Aldo Leopold Wilderness, and Percha Dam, Caballo Lake, and Elephant Butte Lake state parks.

For centuries Indians visited the hot springs at Truth or Consequences for their healing powers. Apaches roamed the Sierra county region by the time Francisco Vázquez de Coronado's expedition arrived in 1540. The Spanish first attempted to mine gold in the Sierra Caballo during the 17th century, and a gold and silver rush came to the Black Range in 1877. The county was established in 1884; the booming mining town of Hillsboro became the county seat.

Truth or Consequences, now the county seat, was a popular retirement town in the late 20th century. Government expenditures, tourism, and agriculture (cattle, milk, vegetables, chiles) also are important to the economy. Area 4,181 square miles (10,828 square km). Pop. (2000) 13,270; (2007 est.) 12,316.