Grand Detour Township is located in Ogle County, Illinois. As of the 2010 census, its population was 698 and it contained 385 housing units. It contains the census-designated place of Grand Detour.
According to the 2010 census, the township has a total area of , of which (or 94.95%) is land and (or 5.05%) is water. The township's land use is predominantly agriculture with single-family residential uses in the southeastern portion of the township.
It is located in a multi-township assessment district with Pine Creek, Eagle Point, and Woosung townships. The township is coterminous with the township's sole voting precinct.