Decatur Township is one of the nine townships in Marion County, Indiana, United States, and part of the consolidated city of Indianapolis. As of the 2020 census, it had a population of 36,951. Located in the southwest corner of the county, the township is home to the Indianapolis International Airport main terminal. It is one of the most rural sections of the county but has seen many new residential and commercial developments. AmeriPlex, one of the largest industrial parks in Indiana, is in Decatur Township. Through the White River, Decatur and Perry townships share the only water boundary among Marion County's townships.