Washington is a borough in Warren County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 7,299, an increase of 838 from the 2010 census count of 6,461, which in turn reflected a decline of 251 from the 6,712 counted in the 2000 census. Washington was incorporated as a borough by an act of the New Jersey Legislature on February 20, 1868, from portions of Washington Township.