Costilla County 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 continuously occupied town in Colorado.
According to the U.S. Census Bureau, the county has a total area of 1,230 square miles (3,200 km), of which 1,227 square miles (3,180 km) is land and 304 square miles (790 km) (0.3%) is water.
As of the census of 2000, there were 3,663 people, 1,503 households, and 1,029 families residing in the county. The population density was 3 people per square mile (1/km²). There were 2,202 housing units at an average density of 2 per square mile (1/km²). The racial makeup of the county was 60.91% White, 0.79% Black or African American, 2.48% Native American, 1.01% Asian, 0.14% Pacific Islander, 29.46% from other races, and 5.21% from two or more races. 67.59% of the population were Hispanic or Latino of any race.
There were 1,503 households out of which 28.50% had children under the age of 18 living with them, 52.60% were married couples living together, 11.30% had a female householder with no husband present, and 31.50% were non-families. 28.10% of all households were made up of individuals and 11.60% had someone living alone who was 65 years of age or older. The average household size was 2.44 and the average family size was 2.98.
In the county, the population was spread out with 25.00% under the age of 18, 6.60% from 18 to 24, 23.30% from 25 to 44, 28.30% from 45 to 64, and 16.80% who were 65 years of age or older. The median age was 42 years. For every 100 females there were 99.80 males. For every 100 females age 18 and over, there were 96.20 males.
The median income for a household in the county was $19,531, and the median income for a family was $25,509, the lowest for Colorado. Males had a median income of $22,390 versus $16,121 for females. The per capita income for the county was $10,748. About 21.30% of families and 26.80% of the population were below the poverty line, including 32.40% of those under age 18 and 23.30% of those age 65 or over.