Jefferson County is a county located in the U.S. state of Iowa. As of the 2020 United States Census, the population was 15,663. The county seat is Fairfield. The county was formed in January 1839, and was named for U.S. President Thomas Jefferson.

Fairfield is a city in, and the county seat of, Jefferson County, Iowa. It has a population of 9,416 people, according to the 2020 census. The median family income is $46,138, with 10% of families below the poverty line.