Ontwa Township is a civil township of Cass County in the U.S. state of Michigan. The population was 6,904 at the 2020 census.
Geography
Ontwa Township is located along the southern border of Cass County, which is also the Michigan–Indiana line. The village of Edwardsburg is in the northwest part of the township. The unincorporated community of Granger, Indiana, borders the township to the southwest.
According to the United States Census Bureau, the township has a total area of , of which is land and , or 7.52%, is water. Several named lakes are in the township, the largest of which are Eagle Lake and Christiana Lake in the northeast. The unincorporated community of Adamsville is near the eastern edge of the township, along Christiana Creek, a south-flowing tributary of the St. Joseph River.