Fairfield Township, one of thirteen townships in Butler County, Ohio, United States, is located in the south central portion of the county. It had a population of 21,373 at the 2010 census, up from 15,571 in 2000, but down sharply from 49,353 in 1990 because of the withdrawal of the city of Fairfield in 1994 and annexations from Fairfield and Hamilton.