Foster township
- County:
- Marion County
- County Seat:
- No
- Area (mi²):
- 35.557
- State:
- Illinois
Foster township is located in Marion County, Illinois. Foster township has a 2026 population of 348. Foster township is currently declining at a rate of -1.14% annually and its population has decreased by -3.87% since the most recent census, which recorded a population of 362 in 2020.
The median household income in Foster township is $129,375 with a poverty rate of 2.82%. The median age in Foster township is 34 years: 35.9 years for males, and 28.7 years for females. For every 100 females there are 100.0 males.