Costilla County According to the U.S. Census Bureau, the county has a total area of 1,230 square miles (3,200 km2), of which 1,227 square miles (3,180 km2) is land and 304 square miles (790 km2) (0.3%) is water.

It is the ninth-least populous of the 64 counties in the U.S. state of Colorado. As of the 2010 census, the population was 3,524. The county seat is San Luis, the oldest town in Colorado.

Costilla County lies in south-central Colorado within the San Luis Valley. It covers nearly 1,227 square miles and ranges in elevation from 8,400 to 10,300 feet. Costilla County borders Conejos County to the west, Alamosa County to the northwest, Huerfano County to the northeast, Las Animas County to the east, and the state of New Mexico to the south. Classified as a high alpine desert, it generally receives fewer than eight inches of precipitation per year, forcing local farmers and ranchers to rely on underground aquifers and mountain reservoirs for water. The county lies to the west of the Sangre de Cristo Mountains, which boasts several peaks above 14,000 feet, including Blanca Peak, Crestone Peak, and Culebra Peak.