Decatur Township is the smallest in geographic size and in population of the nine townships in Marion County, Indiana, United States. As of the 2010 census, it had a population of 32,388. Located on the southwest corner of the county, the township is home to the new Indianapolis International Airport main terminal. The city of Indianapolis and Marion County are a merged unit. Long one of the most rural sections of the county, Decatur Township has seen many new residential and commercial developments.