Fairfield is a city and the county seat of Jefferson County, Iowa, United States. The population was 9,464 in the 2010 census, a decline from 9,509 in the 2000 census.
The area now known as Jefferson County was first settled in 1836, and became Jefferson County in 1839, with the new town of Fairfield as the county seat. The name was suggested by Nancy Bonnifield, one of the settlers, because it aptly described the fair fields of the area. But also author Susan Welty suggests it was a play of words on her own name (bonny field). By 1840 the town had a population of 110 and grew to 650 in 1847. The town was the site of the first and second Iowa State Fair.
Fairfield's library was established in 1853, and was the first library in the state of Iowa. It was first housed in a rented room off the city square. The Carnegie building on the corner of Washington and Court streets became its home in 1893. Then in May 1996, the library moved to its present location on West Adams Street. The library has over 220,000 items and received accreditation from the State Library of Iowa in 2009.