Big Island Township is one of the fifteen townships of Marion County, Ohio, United States.

Located in the western part of the county, it borders the following townships: No municipalities are located in Big Island Township.

Big Island Township was established in 1823, and named for a tract of forest early settlers believed must have been located on an island.

The township is governed by a three-member board of trustees, who are elected in November of odd-numbered years to a four-year term beginning on the following January 1.

There is also an elected township fiscal officer,[5] who serves a four-year term beginning on April 1 of the year after the election, which is held in November of the year before the presidential election.