Flagg Township is located in Ogle County, Illinois. As of the 2010 census, its population was 13,562 and it contained 5,525 housing units.
Flagg Township was named for Richard P. Flagg, a pioneer settler.
According to the 2010 census, the township has a total area of , of which (or 99.75%) is land and (or 0.22%) is water.