Decatur Township is a township in Decatur County, Iowa, United States. As of the 2000 census, its population was 425.
Decatur Township covers an area of 35.77 square miles (92.65 square kilometers); of this, 0.14 square miles (0.35 square kilometers) or 0.38 percent is water. The streams of Long Creek and Marks Branch run through this township.
The township contains six cemeteries: Cash, Decatur City, Palenstine, Shy, Waller and Woodmansee.