Decker Township is located in Richland County, Illinois. As of the 2010 census, its population was 406 and it contained 170 housing units.
Decker Township bears the name of Thomas J. Decker, a local pioneer. Before May 2, 1859, the township was known as Jackson Township.
According to the 2010 census, the township has a total area of , of which (or 99.91%) is land and (or 0.09%) is water.