Pecatonica Township is located in Winnebago County, Illinois. As of the 2010 census, its population was 4,355 and it contained 1,750 housing units. It contains part of the census-designated place of Westlake Village.
According to the 2010 census, the township has a total area of , of which (or 98.99%) is land and (or 1.01%) is water.
Pecatonica Township contains the Pecatonica Wetlands Forest Preserve, a county park.