Fairfield Township is one of fifteen townships in DeKalb County, Indiana. As of the 2020 census, its population was 1,367, representing a tiny decline from 1,368 at the 2010 census, and it contained 661 housing units.
Fairfield Township was founded in 1844.
The Maria and Franklin Wiltrout Polygonal Barn was added to the National Register of Historic Places in 1993.
According to the 2010 census, the township has a total area of , of which (or 99.11%) is land and (or 0.86%) is water. Indian Lake, Lower Story Lake and Upper Story Lake are in this township.