The Black Fork River is a principal tributary of the Mohican River and is 58.4 miles (94 km) long in north-central Ohio in the United States. The Black Fork River drains an area of 351 square miles (909 km) and is part of the Muskingum Watershed Conservancy District.
The Black Fork starts about 7 miles (11 km) west of Mansfield in Richland County, and initially flows northward through the city of Shelby, then eastward across northern Richland County before turning southeast for the remainder of its course through eastern Richland and southern Ashland Counties, past the towns of Perrysville and Loudonville. It joins the Clear Fork in Ashland County to form the Mohican River, about 2 miles (3 km) southwest of Loudonville.
A U.S. Army Corps of Engineers dam in Ashland County, completed in 1936, causes the Black Fork to form Charles Mill Lake.