Washington Township is a township in Warren County, New Jersey, United States. As of the 2010 United States Census, the township’s population was 6,651, reflecting an increase of 403 (+6.5%) from the 6,248 counted in the 2000 Census, which had in turn increased by 881 (+16.4%) from the 5,367 counted in the 1990 Census. It is part of the eastern most region of the Lehigh Valley. It is one of six municipalities in New Jersey under the name Washington (one of which is a borough, five of which are townships). Washington Township, Warren County completely surrounds the borough of Washington. Mansfield Township, also in Warren County, borders both this municipality and another Washington Township in Morris County
Washington Township was incorporated as a township by an Act of the New Jersey Legislature on April 9, 1849, from portions of Mansfield Township. Portions of the township were taken on February 20, 1868, to create Washington Borough.