Sussex is a borough in Sussex County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 2,024, a decrease of 106 from the 2010 census count of 2,130, which in turn reflected a decline of 15 from the 2,145 counted in the 2000 census. Sussex was incorporated as a borough by an act of the New Jersey Legislature on October 14, 1891, as Deckertown, from portions of Wantage Township. The borough's original name was for settler Peter Decker.