Washington Township is one of thirteen townships in Fremont County, Iowa, United States. As of the 2010 census, its population was 350 and it contained 155 housing units.
As of the 2010 census, Washington Township covered an area of ; of this, (98.79 percent) was land and (1.21 percent) was water.
Lower Hamburg Island, an island of the Missouri River, is located in the southwesternmost portion of the township but is mostly located in Atchison County, Missouri. It contains the southwesternmost point in Iowa.
The township contains Austin Cemetery, Baker Cemetery, Brown Cemetery, Buckham Cemetery, Frost Cemetery, Hamburg Cemetery, Lovelady Cemetery, Means Cemetery, Saint Marys Cemetery and Slusher Cemetery.