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. The county seat is San Luis, the oldest continuously occupied town in Colorado.

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. 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 been colonized. 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. 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.

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.

• Huerfano County - northeast
• Las Animas County - east
• Colfax County, New Mexico - southeast
• Taos County, New Mexico - south
• Conejos County - west
• Alamosa County - northwest

• U.S. Highway 160
• State Highway 142
• State Highway 159

• San Isabel National Forest

• Fort Garland State History Museum
• Los Caminos Antiguos Scenic and Historic Byway
• Old Spanish National Historic Trail

Historical population

Census	Pop.	Note	%±
1870	1,779		—
1880	2,879		61.8%
1890	3,491		21.3%
1900	4,632		32.7%
1910	5,498		18.7%
1920	5,032		−8.5%
1930	5,779		14.8%
1940	7,533		30.4%
1950	6,067		−19.5%
1960	4,219		−30.5%
1970	3,091		−26.7%
1980	3,071		−0.6%
1990	3,190		3.9%
2000	3,663		14.8%
2010	3,524		−3.8%
2020	3,499		−0.7%
2023 (est.)	3,628		3.7%
U.S. Decennial Census 1790-1960 1900-1990 1990-2000 2010-2020

As of the census 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/km). There were 2,202 housing units at an average density of 2 per square mile (0.77/km). 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, 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.

United States presidential election results for Costilla County, Colorado

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2020	741	35.39%	1,311	62.61%	42	2.01%
2016	588	31.82%	1,125	60.88%	135	7.31%
2012	446	24.28%	1,340	72.95%	51	2.78%
2008	415	24.45%	1,245	73.36%	37	2.18%
2004	566	32.16%	1,170	66.48%	24	1.36%
2000	504	30.58%	1,054	63.96%	90	5.46%
1996	333	20.29%	1,168	71.18%	140	8.53%
1992	366	20.88%	1,180	67.31%	207	11.81%
1988	454	28.72%	1,120	70.84%	7	0.44%
1984	621	38.07%	997	61.13%	13	0.80%
1980	489	30.89%	1,036	65.45%	58	3.66%
1976	392	26.96%	1,033	71.05%	29	1.99%
1972	602	42.16%	744	52.10%	82	5.74%
1968	477	32.19%	933	62.96%	72	4.86%
1964	299	18.82%	1,284	80.81%	6	0.38%
1960	637	31.83%	1,351	67.52%	13	0.65%
1956	958	42.50%	1,256	55.72%	40	1.77%
1952	1,070	43.73%	1,369	55.95%	8	0.33%
1948	921	36.37%	1,563	61.73%	48	1.90%
1944	896	37.09%	1,515	62.71%	5	0.21%
1940	1,121	39.40%	1,698	59.68%	26	0.91%
1936	930	37.14%	1,518	60.62%	56	2.24%
1932	707	31.92%	1,475	66.59%	33	1.49%
1928	657	37.33%	1,070	60.80%	33	1.88%
1924	755	48.37%	665	42.60%	141	9.03%
1920	778	49.52%	750	47.74%	43	2.74%
1916	579	34.65%	1,028	61.52%	64	3.83%
1912	1,072	55.06%	567	29.12%	308	15.82%
1908	1,051	64.60%	559	34.36%	17	1.04%
1904	917	63.46%	506	35.02%	22	1.52%
1900	884	65.43%	453	33.53%	14	1.04%
1896	96	3.85%	2,388	95.83%	8	0.32%
1892	526	63.45%	0	0.00%	303	36.55%
1888	507	56.46%	383	42.65%	8	0.89%
1884	498	50.71%	484	49.29%	0	0.00%
1880	334	46.84%	379	53.16%	0	0.00%

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.

• Blanca
• San Luis

• Fort Garland
• San Acacio

• Chama
• Garcia
• Jaroso
• Mesita
• Russell

• 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

• Official website
• Colorado County Evolution by Don Stanwyck
• Colorado Historical Society
• San Luis Valley Information Center

37°17′N 105°26′W / 37.28°N 105.43°W / 37.28; -105.43