St. Joseph County is a county located in the U.S. state of Michigan, on the central southern border with Indiana. As of the 2020 United States Census, the population was 60,939. The county seat is Centreville.

French colonists in the late 17th century were the first Europeans to explore this territory, and they named the St. Joseph River for the patron saint of New France. This area was not part of the United States until after the American Revolutionary War. After the 1821 Treaty of Chicago was signed, regional tribes of the indigenous peoples ceded much land to the United States, opening the area for American settlement. The county was set off and organized by the Michigan Territory legislature in 1829; it was named for the river.

The area is home to the oldest and largest Amish community in Michigan.

St. Joseph County comprises the Sturgis, MI Micropolitan Statistical Area and is included in the Kalamazoo-Battle Creek-Portage, MI Combined Statistical Area.

This area was settled by members of the three Algonquian-speaking tribes of the Council of Three Fires: the Potawatomi, Odawa, and Chippewa (known as Ojibwa in Canada). French explorers in a party led by Father Hennepin came upriver from Lake Michigan in 1679. A Jesuit mission was established near where the French later built Fort St. Joseph, and they named the waterway as the St. Joseph River.

After the United States and tribal representatives made the 1821 Treaty of Chicago, much of the land in this region was ceded by the tribes to Michigan Territory, so more settlers came from Detroit, Monroe and later from New England and upstate New York. The first were ethnic French. The Godfroy brothers of Detroit built a trading post south of the river, near the later Marantette House site. They appointed Frances Mouton as an agent to trade with the natives of the Nottawaseepe (sippi) settlement. They had another post nearby where Patrick Marantette, also from Detroit, first worked as an agent. He ended up settling in and marrying Mouton's daughter Francis. Theirs was the first marriage in the settlement, and their daughter the first European American born here, in 1836.

According to the U.S. Census Bureau, the county has a total area of 521 square miles (1,350 km), of which 501 square miles (1,300 km) is land and 20 square miles (52 km) (3.9%) is water. It is the fourth-smallest county in Michigan by total area. The entire county lies in the Saint Joseph River watershed.

• US 12 runs east–west across the lower portion of the county and passes White Pigeon and Sturgis.
• US 131 runs north–south through the western portion of the county and passes Three Rivers, Constantine, and White Pigeon.
• 
  Bus. US 131 runs north–south through eastern Three Rivers.
• M-60 runs east-northeast through the upper portion of the county and passes Three Rivers, Parkville, Mendon, Leonidas.
• M-66 enters the southeastern portion of the county from Star Mill, Indiana, and runs north to an intersection with M-60, two miles (3.2 km) east of Mendon.
• M-86 runs east–west through the center of the county, entering at Colon, passing Nella and Centreville to an intersection with M-60 at Three Rivers.
• M-103 enters the southwestern tip of the county and runs north two miles (3.2 km) to intersection with US 12 near the western county line.
• M-216 enters the northwestern portion of the county from Marcellus and runs east to an intersection with US 131 four miles (6.4 km) north of Three Rivers.

Strictly speaking, the Indiana Toll Road does not enter St. Joseph County, but it has an interchanges with US 131 barely within Indiana. Although M-66 does not quite reach the Toll Road, the toll road interchange is in clear sight from M-66 before it becomes Indiana State Road 9.

Historical population

Census	Pop.	Note	%±
1830	1,313		—
1840	7,068		438.3%
1850	12,725		80.0%
1860	21,262		67.1%
1870	26,275		23.6%
1880	26,626		1.3%
1890	23,356		−12.3%
1900	23,889		2.3%
1910	25,499		6.7%
1920	26,818		5.2%
1930	30,618		14.2%
1940	31,749		3.7%
1950	35,071		10.5%
1960	42,332		20.7%
1970	47,392		12.0%
1980	56,083		18.3%
1990	58,913		5.0%
2000	62,422		6.0%
2010	61,295		−1.8%
2020	60,939		−0.6%
2023 (est.)	60,878		−0.1%
US Decennial Census 1790-1960 1900-1990 1990-2000 2010-2018

The 2010 census indicates St. Joseph County had a 2010 population of 61,295. This decrease of -1,127 people from the 2000 United States Census represents a -1.8% population change (decrease) in that decade. In 2010 there were 23,244 households and 16,275 families in the county. The population density was 122.4 per square mile (47.3 per square kilometer). There were 27,778 housing units at an average density of 55.5 per square mile (21.4/km). The racial and ethnic makeup of the county was 88.0% White, 2.5% Black or African American, 0.4% Native American, 0.7% Asian, 6.6% Hispanic or Latino, 0.1% from other races, and 1.8% from two or more races.

There were 23,244 households, out of which 33.0% had children under the age of 18 living with them, 52.6% were husband and wife families, 11.7% had a female householder with no husband present, 30.0% were non-families, and 24.8% were made up of individuals. The average household size was 2.60 and the average family size was 3.08.

The county population contained 25.9% under age of 18, 8.1% from 18 to 24, 23.8% from 25 to 44, 27.4% from 45 to 64, and 14.9% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 97.9 males. For every 100 females age 18 and over, there were 95.9 males.

The 2010 American Community Survey 3-year estimate indicates the median income for a household in the county was $43,964 and the median income for a family was $52,600. Males had a median income of $30,517 versus $16,388 for females. The per capita income for the county was $19,737. About 1.8% of families and 16.3% of the population were below the poverty line, including 22.3% of those under the age 18 and 12.3% of those age 65 or over.

St. Joseph County has long been reliably Republican. Since 1884, the Republican Party nominee has carried 79% of the elections (27 of 34).

United States presidential election results for St. Joseph County, Michigan

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2020	18,127	64.78%	9,262	33.10%	592	2.12%
2016	14,884	62.10%	7,526	31.40%	1,557	6.50%
2012	12,978	55.36%	10,112	43.13%	355	1.51%
2008	12,886	50.00%	12,322	47.81%	563	2.18%
2004	15,340	60.78%	9,648	38.23%	251	0.99%
2000	12,906	58.60%	8,574	38.93%	544	2.47%
1996	9,764	46.96%	8,529	41.02%	2,501	12.03%
1992	9,836	41.03%	7,817	32.61%	6,318	26.36%
1988	13,084	64.79%	7,017	34.74%	95	0.47%
1984	15,405	72.34%	5,795	27.21%	96	0.45%
1980	13,631	63.19%	6,318	29.29%	1,621	7.52%
1976	11,784	61.07%	7,306	37.86%	205	1.06%
1972	12,438	69.15%	5,119	28.46%	431	2.40%
1968	10,445	59.20%	5,413	30.68%	1,787	10.13%
1964	7,307	43.96%	9,284	55.85%	32	0.19%
1960	12,337	69.13%	5,445	30.51%	65	0.36%
1956	12,328	74.11%	4,242	25.50%	64	0.38%
1952	12,191	72.19%	4,509	26.70%	187	1.11%
1948	8,166	65.66%	3,928	31.59%	342	2.75%
1944	9,785	69.45%	4,235	30.06%	69	0.49%
1940	10,025	66.32%	5,045	33.38%	45	0.30%
1936	7,160	51.85%	6,048	43.80%	601	4.35%
1932	5,626	43.78%	6,917	53.82%	309	2.40%
1928	8,781	76.05%	2,698	23.37%	67	0.58%
1924	6,633	65.43%	2,649	26.13%	855	8.43%
1920	6,035	66.59%	2,725	30.07%	303	3.34%
1916	3,132	45.01%	3,567	51.26%	260	3.74%
1912	1,224	19.03%	2,388	37.13%	2,820	43.84%
1908	3,466	52.93%	2,770	42.30%	312	4.76%
1904	3,649	59.80%	2,176	35.66%	277	4.54%
1900	3,178	48.19%	3,283	49.78%	134	2.03%
1896	3,184	43.76%	3,968	54.54%	124	1.70%
1892	2,824	43.93%	2,441	37.97%	1,163	18.09%
1888	3,372	48.36%	3,217	46.14%	383	5.49%
1884	3,261	46.89%	3,554	51.11%	139	2.00%

The county government operates the jail, operates the major local courts, records deeds, mortgages, and vital records, administers public health regulations, and participates with the state in the provision of social services. The county board of commissioners controls the and has limited authority to make laws or ordinances. In Michigan, most local government functions – police and fire, building and zoning, tax assessment, street maintenance, etc. – are the responsibility of individual cities and townships.

(Information verified with county website as of August 2024)

• Sturgis
• Three Rivers

• List of Michigan State Historic Sites in St. Joseph County, Michigan
• National Register of Historic Places listings in St. Joseph County, Michigan

• St. Joseph County website
• "Bibliography on St. Joseph County". Clarke Historical Library, Central Michigan University.

41°55′N 85°32′W / 41.92°N 85.53°W / 41.92; -85.53