According to the U.S. Census Bureau, Salem Township has an area of 24.8 square miles (64.2 km2).

[5] Organized in 1836, the township was originally part of Mercer County and included portions of what are now Jennings Township in Van Wert County and Spencer Township in Allen County.

[6] After the creation of Auglaize County in 1848, the remaining portions of Salem Township were joined with the northern portions of what was then Wayne Township in Mercer County.

[7] 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.

The northern section of the township is served by the Spencerville (45887) post office, with the southern section served by the Saint Marys (45884) and the Mendon (45862) post offices.