Jefferson County is a county located in the U.S. state of Kansas. Its county seat and most populous city is Oskaloosa. As of the 2020 census, the county population was 18,368. The county was named after Thomas Jefferson, the 3rd president of the United States.

Oskaloosa is a city in and the county seat of Jefferson County, Kansas, United States. As of the 2020 census, the population of the city was 1,110.