Forrest Township is located in Livingston County, Illinois. As of the 2010 census, its population was 1,605 and it contained 627 housing units.
Forrest Township was created from Chatsworth Township in 1861 as Forrestville Township. The name was changed to Forrest Township in 1865.
According to the 2010 census, the township has a total area of , all land.