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 (-0.7%) from the 2,145 counted in the 2000 Census, which had in turn declined by 56 (-2.5%) 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.
A joint commission of residents of both Sussex and Wantage had recommended that the two communities should be consolidated to form what would be called the Township of Sussex-Wantage, which would operate within the Faulkner Act under the council-manager form of government, with a mayor and a six-member township council, and that voters in both municipalities should approve a referendum to be held on November 3, 2009. The committee noted that the two municipalities share common issues, schools, library and community services and that the artificial nature of the octagonal Sussex border often made it hard to distinguish between the two. The efforts at consolidation with surrounding Wantage Township ended in November 2009 after Wantage voters rejected the merger despite support from Sussex borough residents.
According to the United States Census Bureau, Sussex borough had a total area of 0.62 square miles (1.62 km2), including 0.59 square miles (1.54 km2) of land and 0.03 square miles (0.08 km2) of water (5.00%). It is approximately 400 to 450 feet (120 to 140 m) above sea level.
The borough is completely surrounded by Wantage Township, making it part of 21 pairs of "doughnut towns" in the state, where one municipality entirely surrounds another.
The borough is in the watershed of the Wallkill River (which flows north, and empties into the Rondout Creek, which flows into the Hudson River near Kingston, New York) and its tributary Glen Brook, which near Sussex forms a small body of water called Clove Lake, part of which is within the borough.
Due to its inland location and elevation, Sussex has a climate much cooler than most of the state, classified as humid continental (Köppen Dfb), with cold, moderately snowy winters, and very warm, humid summers. It is part of USDA Plant Hardiness Zone 6.