Newcastle Township is one of eight townships in Fulton County, Indiana. As of the 2020 census, its population was 1,390 (slightly down from 1,398 at 2010) and it contained 552 housing units.
The John Haimbaugh Round Barn was listed on the National Register of Historic Places in 1993.
According to the 2010 census, the township has a total area of , of which (or 99.79%) is land and (or 0.24%) is water. Barr Lake is in this township.
The township contains four cemeteries: Hamlett, Reister, Sycamore and Yellow Creek.