Jefferson County is a county in the U.S. state of Wisconsin. As of the 2020 census, the population was 84,900.^[1] Its county seat is Jefferson.^[2] Jefferson County comprises the Watertown-Fort Atkinson, WI Micropolitan Statistical Area, which is also included in the Milwaukee-Racine-Waukesha, WI Combined Statistical Area.

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Jefferson County
County
Jefferson County courthouse
Location within the U.S. state of Wisconsin
Wisconsin's location within the U.S.
Coordinates: 43°01′N 88°47′W
Country	United States
State	Wisconsin
Founded	1839
Named for	Thomas Jefferson
Seat	Jefferson
Largest city	Watertown
Area
• Total	583 sq mi (1,510 km2)
• Land	556 sq mi (1,440 km2)
• Water	26 sq mi (70 km2)  4.5%
Population  (2020)[1]
• Total	84,900
• Density	152.6/sq mi (58.9/km2)
Time zone	UTC−6 (Central)
• Summer (DST)	UTC−5 (CDT)
Congressional district	5th
Website	www.jeffersoncountywi.gov

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Jefferson County was created in 1836 as part of Wisconsin Territory and was organized in 1839.^[3] Jefferson County was founded by "Yankee" settlers from New England.^[4] It was named after Jefferson County, New York, where some of the original settlers came from.^[5] The town of Watertown, Wisconsin, was named after Watertown, New York, in Jefferson County, New York.

According to the U.S. Census Bureau, the county has an area of 583 square miles (1,510 km²), of which 556 square miles (1,440 km²) is land and 26 square miles (67 km²) (4.5%) is water.^[6]

As of the census of 2020,^[1] the population was 84,900. The population density was 152.6 people per square mile (58.9 people/km²). There were 36,376 housing units at an average density of 65.4 units per square mile (25.3 units/km²). The racial makeup of the county was 88.5% White, 1.0% Black or African American, 0.8% Asian, 0.4% Native American, 3.4% from other races, and 5.8% from two or more races. Ethnically, the population was 8.3% Hispanic or Latino of any race.

As of the census^[12] of 2010, there were 83,686 people, 32,117 households, and 21,872 families residing in the county. The population density was 133 people per square mile (51 people/km²). There were 30,092 housing units at an average density of 54 units per square mile (21 units/km²). The county's racial makeup was 96.34% White, 0.28% Black or African American, 0.34% Native American, 0.45% Asian, 0.02% Pacific Islander, 1.65% from other races, and 0.93% from two or more races. 4.09% of the population were Hispanic or Latino of any race. 55.1% were of German, 6.1% Norwegian, 6.0% Irish and 5.1% American ancestry.

There were 28,205 households, out of which 33.20% had children under the age of 18 living with them, 58.50% were married couples living together, 8.20% had a female householder with no husband present, and 29.50% were non-families. 23.60% of all households were made up of individuals, and 9.60% had someone living alone who was 65 years of age or older. The average household size was 2.55 and the average family size was 3.02.

In the county, the population was spread out, with 25.20% under the age of 18, 8.50% from 18 to 24, 30.40% from 25 to 44, 23.20% from 45 to 64, and 12.60% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 98.40 males. For every 100 females age 18 and over, there were 96.80 males.

In 2017, there were 861 births, giving a general fertility rate of 53.7 births per 1000 women aged 15–44, the eleventh lowest rate out of all 72 Wisconsin counties.^[13]

The Dwight Foster Public Library, which serves as the resource library for the county,^[14] had a total service population of 19,095 in 2010.^[15] It is a member of the Bridges Library System.

The County Board of Supervisors, with 30 members, serves as the legislative body for the county. There are seven elected officials in addition to the County Board of Supervisors. County-wide partisan elections are held in November. The County Board's members are elected for two-year terms during a spring non-partisan election. The County Board is responsible for the county administrator.^{[clarification needed]} The County Board elects a chairman, vice-chairman, and second vice-chairman.

Jefferson County has been primarily Republican since 1940. Only three Democratic presidential candidates have won the county since then, in 1964, 1996, and 2008. Lyndon B. Johnson in 1964 is the last Democratic Party candidate to win a majority of the county's votes.

School districts (all K-12) include:^[17]

• National Register of Historic Places listings in Jefferson County, Wisconsin