Modoc, Illinois is an unincorporated community in Randolph County, Illinois located 4 miles southeast of Prairie du Rocher, Illinois under the bluffs of the Mississippi River. The Modoc Rock Shelter, a U.S. National Historic Landmark, is located near Modoc and the Ste. Genevieve-Modoc Ferry across the Mississippi River connects Modoc and the surrounding communities with Ste. Genevieve, Missouri. The population as of the 2000 census was 221.