Big Horn County is a county located in the U.S. state of Montana. As of the 2020 census, the population was 13,124. The county seat is Hardin. The county, like the river and the mountain range, is named after the bighorn sheep in the Rocky Mountains. The county was founded in 1913. It is located on the south line of the state.

Most of the area is part of the Crow Indian Reservation. Reservation poverty affects the county, which is the second-poorest county in the state.

On June 25, 1876, the Battle of Little Bighorn began. The Battle is also known as the Battle of the Greasy Grass to the Lakota and other Plains Indians.

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The county has several jurisdictions, each with its own regulations and law enforcement agencies. The Crow and Northern Cheyenne Indian Nations are administered by the tribes. Little Bighorn Battlefield and the Big Horn Canyon National Recreation Area are regulated by the National Park Service. The remainder of the county falls under the State of Montana.

According to the United States Census Bureau, the county has a total area of 5,015 square miles (12,990 km), of which 4,995 square miles (12,940 km) is land and 19 square miles (49 km) (0.4%) is water. It is the fifth-largest county in Montana by land area. Most of the county's land area is Indian reservations: The Crow Indian Reservation covers 64.2 percent of its area, while the Northern Cheyenne Indian Reservation covers another 6.37 percent.

The county is home to the Big Horn, Pryor and Wolf mountain ranges.

• Bighorn Canyon National Recreation Area (part)
• Little Bighorn Battlefield National Monument

According to the Köppen Climate Classification system, Big Horn County has a semi-arid climate, abbreviated "BSk" on climate maps.

Big Horn County is generally Democratic, owing largely to its majority Native American population. It is distinguished from most other counties in rural Montana, which often lean heavily Republican.

United States presidential election results for Big Horn County, Montana

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2020	2,207	46.10%	2,491	52.04%	89	1.86%
2016	1,853	43.73%	2,094	49.42%	290	6.84%
2012	1,667	36.04%	2,882	62.30%	77	1.66%
2008	1,628	31.19%	3,516	67.37%	75	1.44%
2004	2,028	47.04%	2,215	51.38%	68	1.58%
2000	1,651	39.68%	2,345	56.36%	165	3.97%
1996	1,336	31.52%	2,453	57.87%	450	10.62%
1992	1,377	31.34%	2,154	49.02%	863	19.64%
1988	1,711	42.95%	2,233	56.05%	40	1.00%
1984	2,390	46.77%	2,681	52.47%	39	0.76%
1980	1,730	46.32%	1,644	44.02%	361	9.67%
1976	1,615	44.49%	1,962	54.05%	53	1.46%
1972	2,148	56.17%	1,552	40.59%	124	3.24%
1968	1,789	53.87%	1,319	39.72%	213	6.41%
1964	1,481	37.09%	2,509	62.83%	3	0.08%
1960	1,724	53.47%	1,497	46.43%	3	0.09%
1956	1,739	56.44%	1,342	43.56%	0	0.00%
1952	2,165	65.91%	1,114	33.91%	6	0.18%
1948	1,334	49.37%	1,328	49.15%	40	1.48%
1944	1,394	51.73%	1,289	47.83%	12	0.45%
1940	1,616	45.39%	1,926	54.10%	18	0.51%
1936	1,087	34.12%	2,037	63.94%	62	1.95%
1932	957	36.42%	1,637	62.29%	34	1.29%
1928	1,274	55.46%	1,017	44.28%	6	0.26%
1924	1,082	57.74%	327	17.45%	465	24.81%
1920	1,062	66.17%	475	29.60%	68	4.24%
1916	497	39.63%	740	59.01%	17	1.36%

Historical population

Census	Pop.	Note	%±
1920	7,015		—
1930	8,543		21.8%
1940	10,419		22.0%
1950	9,824		−5.7%
1960	10,007		1.9%
1970	10,057		0.5%
1980	11,096		10.3%
1990	11,337		2.2%
2000	12,671		11.8%
2010	12,865		1.5%
2020	13,124		2.0%
2022 (est.)	12,851		−2.1%
U.S. Decennial Census 1790–1960, 1900–1990, 1990–2000, 2010–2020

As of the 2020 census, there were 13,124 people living in the county.

As of the 2010 census, there were 12,865 people, 4,004 households, and 2,970 families living in the county. The population density was 2.6 inhabitants per square mile (1.0/km). There were 4,695 housing units at an average density of 0.9 units per square mile (0.35 units/km). The racial makeup of the county was 64.3% American Indian, 31.4% white, 0.5% Asian, 0.2% black or African American, 1.0% from other races, and 2.6% from two or more races. Those of Hispanic or Latino origin made up 4.0% of the population. In terms of ancestry, 12.1% were American, and 10.3% were German.

Of the 4,004 households, 45.6% had children under the age of 18 living with them, 48.8% were married couples living together, 17.3% had a female householder with no husband present, 25.8% were non-families, and 23.0% of all households were made up of individuals. The average household size was 3.18 and the average family size was 3.77. The median age was 30.5 years.

The median income for a household in the county was $36,550 and the median income for a family was $41,985. Males had a median income of $32,216 versus $27,917 for females. The per capita income for the county was $15,066. About 20.7% of families and 23.5% of the population were below the poverty line, including 32.4% of those under age 18 and 15.4% of those age 65 or over.

Coal mining and agriculture play major roles in Big Horn County's economy. Farms and ranches in the county produce mainly beef cattle, sugar beets, alfalfa, and small grains.

• Hardin (county seat)

• Lodge Grass

• Corinth
• Kirby
• Quietus

• List of lakes of Big Horn County, Montana
• List of mountains in Big Horn County, Montana
• National Register of Historic Places listings in Big Horn County, Montana

• Home page

45°26′N 107°29′W / 45.43°N 107.49°W / 45.43; -107.49