Washington Township is a township in southwestern Morris County, in the U.S. state of New Jersey. As of the 2020 United States census, the township's population was 18,197, a decrease of 336 (−1.8%) from the 2010 census count of 18,533, which in turn reflected an increase of 941 (+5.3%) from the 17,592 counted in the 2000 census.