Costilla County (Spanish for "rib") is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 3,499.^[1] The county seat is San Luis,^[2] the oldest continuously occupied town in Colorado.

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Costilla County
County
The Costilla County Courthouse in San Luis
Location within the U.S. state of Colorado
Colorado's location within the U.S.
Coordinates: 37°17′N 105°26′W
Country	United States
State	Colorado
Founded	November 1, 1861
Seat	San Luis
Largest town	San Luis
Area
• Total	1,230 sq mi (3,200 km2)
• Land	1,227 sq mi (3,180 km2)
• Water	3.4 sq mi (9 km2)  0.3%
Population  (2020)
• Total	3,499[1]
• Density	2.9/sq mi (1.1/km2)
Time zone	UTC−7 (Mountain)
• Summer (DST)	UTC−6 (MDT)
Congressional district	3rd
Website	costillacounty.colorado.gov
Colorado's first permanent settlement

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On July 8, 1694, Spanish Conquistador Don Diego de Vargas and his army, two weeks before the Battle of Astialakwa, reached Costilla County. Diego Vargas is not the first Spaniard in Colorado. Juan de Archuleta led an expedition into Colorado in 1664 - but his expedition is the first traceable Spanish expedition into Colorado.^[3] In 1647, Governor Luis Rosas fought with the Utes in northern New Mexico. While Rosa came near Colorado, it has not been verified he actually did.

Costilla County was the first area of Colorado to have civilized settlement. The county made up the major part of the Sangre de Cristo Land Grant awarded by the government of New Mexico to the Carlos Beaubien family in 1843. Hispanic settlers from Taos, New Mexico, officially established San Luis on April 9, 1851.^[4] Costilla County was one of the original 17 counties created by the Territory of Colorado on November 1, 1861. The county was named for Costilla Creek. San Miguel was originally designated the county seat, but the county government was moved to San Luis in 1863. (In 1869, surveys placed San Miguel in the New Mexico Territory.)

The county's original boundaries extended 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.^[5]

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.^[6]

As of the census^[12] of 2000, there were 3,663 people, 1,503 households, and 1,029 families living in the county. The population density was 3 people per square mile (1.2 people/km²). There were 2,202 housing units at an average density of 2 per square mile (0.77/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.

Costilla County tends to favor the Democratic candidate in Presidential elections. The last Republican to carry the county was Calvin Coolidge in 1924,^[13] and the last to gain an absolute majority William Howard Taft in 1912 – an era when most votes in these high valley counties were done for the voters by political machines. In the last eleven Presidential elections the Democratic candidate has consistently received over sixty percent of the county's vote and four times won over seventy percent.

In Colorado's first elections as a state in 1876, Auguste Lacome (D) ran against William H. Meyer (R) for State Senate in Costilla County, then Colorado's 18th District. Meyer would later become the Lt. Governor of Colorado. Votes cast for “Locome” and “Lacompte” were included in the count for Lacome. Meyer carried the election 349–204.

It is part of Colorado's 3rd congressional district, which has a Cook Partisan Voting Index of R+5 and is represented by Republican Lauren Boebert. In the Colorado Senate it is in District 35 and is represented by Rod Pelton. In the Colorado House of Representatives it is in District 62 and is represented by Democrat Matthew Martinez.

• Bibliography of Colorado
• Geography of Colorado
• History of Colorado
  • Auguste Lacome
  • San Luis de la Culebra
  • San Miguel
  • National Register of Historic Places listings in Costilla County, Colorado
• Index of Colorado-related articles
• List of Colorado-related lists
  • List of counties in Colorado
• Outline of Colorado