Sussex is a borough in Sussex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 2,130, reflecting a decline of 15 from the 2,145 counted in the 2000 Census, which had in turn declined by 56 from the 2,201 counted in the 1990 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. The borough was renamed Sussex on March 2, 1902. The county and borough are named for the historic county of Sussex in England.