 Duplain Township is located in the northeast corner of Clinton County and is bordered on the east by Shiawassee County and on the north by Gratiot County.According to the United States Census Bureau, the township has a total area of , of which is land and , or 0.83%, is water.As of the census of 2000, there were 2,329 people, 866 households, and 637 families residing in the township.