Hopatcong /hoʊpætkɑːnɡ/ is a borough in Sussex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 15,147, reflecting a decline of 741 from the 15,888 counted in the 2000 Census, which had in turn increased by 302 from the 15,586 counted in the 1990 Census.