Marion Township is one of the fourteen townships of Mercer County, Ohio, United States. The 2020 census found 3,322 people in the township.

Mercer County is located in the U.S. state of Ohio. As of the 2020 census, the population was 42,528. Its county seat is Celina. The county was created in 1820 and later organized in 1824. It is named for Hugh Mercer, an officer in the American Revolutionary War.