Cedar Bog State Nature Preserve is a fen left behind by the retreating glaciers of the Wisconsin glaciation about 12,000-18,000 years ago.

Cedar Bog is located in Champaign County, Ohio, United States, near the city of Urbana.

The sedges and other plants that grow here left behind by the last glacier were the food for mastodons and giant sloths that once roamed the earth.

Cedar Bog is also the home of the endangered spotted turtle, massasauga rattlesnake, and Milbert's tortoise-shell butterfly.

Cedar Bog was purchased in 1942 by the State of Ohio using public funds for the purpose of setting it aside as a natural area.