Forreston Township is located in Ogle County, Illinois. As of the 2010 census, its population was 2,080 and it contained 885 housing units. Forreston Township was formed from a portion of Brookville Township on March 4, 1857. The village of Forreston and the unincorporated community of Baileyville are located within it.
According to the 2010 census, the township has a total area of , of which (or 99.94%) is land and (or 0.06%) is water.