Foster Township is located in Madison County, Illinois, in the United States. As of the 2010 census, its population was 4,091 and it contained 1,663 housing units.
Foster is the name of Oliver Foster, a pioneer settler and native of New Hampshire.
According to the 2010 census, the township has a total area of , of which (or 98.47%) is land and (or 1.53%) is water.