Fairfield is a city in, and the county seat of, Jefferson County, Iowa, United States. 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. It became the county seat in 1839 with 110 residents and grew to 650 by 1847. Its library was established in 1853, and it held its first fair in 1854. Early architecture in Fairfield includes work by George Franklin Barber and Barry Byrne.