Jefferson County is a county located in the state of Wisconsin. As of 2000, the population is 74,021. Its county seat is Jefferson⁶.

Geography - According to the U.S. Census Bureau, the county has a total area of 1,509 km² (583 mi²). 1,443 km² (557 mi²) of it is land and 67 km² (26 mi²) of it is water. The total area is 4.42% water.

Demographics - As of the census² of 2000, there are 74,021 people, 28,205 households, and 19,884 families residing in the county. The population density is 51/km² (133/mi²). There are 30,092 housing units at an average density of 21/km² (54/mi²). The racial makeup of the county is 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 are Hispanic or Latino of any race.

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

In the county, the population is 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 are 65 years of age or older. The median age is 37 years. For every 100 females there are 98.40 males. For every 100 females age 18 and over, there are 96.80 males.