Greenwich Township (pronounced GREEN-witch) is a township in Warren County, New Jersey, United States. As of the 2010 United States Census, the township's population was 5,712, reflecting an increase of 1,347 (+30.9%) from the 4,365 counted in the 2000 Census, which had in turn increased by 2,466 (+129.9%) from the 1,899 counted in the 1990 Census. The township is located in the far eastern region of the Lehigh Valley.
Greenwich Township has a long history passing through as part of most of Northwestern New Jersey's counties. The township was first mentioned in official documents on October 9, 1738, as a part of Hunterdon County. On March 15, 1739, it became part of the newly formed Morris County. On January 22, 1750, portions of the township were taken to form Hardwick Township. On June 8, 1753, Sussex County was created, and Greenwich Township was shifted again. Portions of the township were taken on May 30, 1754, to form both Mansfield Township and Oxford Township. The township was incorporated by an Act of the New Jersey Legislature on February 21, 1798. It found its current home when Warren County was formed on November 20, 1824. On April 8, 1839, portions of the township were taken to create Franklin Township and Harmony Township. On March 7, 1851, Phillipsburg was created from parts of Greenwich, and Pohatcong Township went off on its own as of January 1, 1882.
Greenwich CDP (with a 2010 Census population of 2,755), Stewartsville (with 349 residents in 2010) and Upper Stewartsville (2010 population of 212) are census-designated places (CDPs) and unincorporated communities located within the township.