Costilla County is the ninth least populous of the 64 counties in the state of Colorado of the United States. The county population was 3,663 at U.S. Census 2000. The county seat is San Luis, the oldest town in Colorado.
Costilla County was the first area of Colorado to be settled by European-Americans. Hispanic settlers from Taos, New Mexico, officially established San Luis on April 9, 1851. Costilla County was one of the original 17 counties created by the Territory of Colorado on November 1, 1861. The county was named for the Costilla River. Although San Miguel was originally designated the county seat, the county government was moved to San Luis in 1863.
The county's original boundaries had the county extend over much of south-central Colorado. Much of the northern portion became part of Saguache County in 1866, and the western portions were folded into Hinsdale and Rio Grande counties in 1874. Costilla County arrived at its modern boundaries in 1913 when Alamosa County was created from its northwest portions.