Stanhope is a borough in Sussex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 3,610, reflecting an increase of 26 from the 3,584 counted in the 2000 Census, which had in turn increased by 191 from the 3,393 counted in the 1990 Census.Stanhope was formed by an act of the New Jersey Legislature on March 24, 1904, from portions of Byram Township.