As of the 2010 census, its population was 379 and it contained 193 housing units. Foster Township is mainly a farming community with corn, wheat and soy beans being the major crop. It no longer has towns or villages within its boundaries.
Foster Township (T4N R2E) is located in Marion County, Illinois. It is centered at 38ð47'N 88ð59'W (38.783, -88.981). It is located in the northern part of Marion County, bordering Fayette County on its north, and the townships of Kinmundy to its east, Tonti on its south, and Patoka to its west.
Wildlife includes deer, turkey, rabbits and raccoons and an occasional coyote or bobcat. The township consists of flat prairie on the north and west and timber and hills in the south and east.
According to the 2010 census, the township has a total area of , of which (or 99.89%) is land and (or 0.11%) is water.
There are nine cemeteries in the township: