Andover is a borough in Sussex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 606, reflecting a decline of 52 from the 658 counted in the 2000 Census, which had in turn declined by 42 from the 700 counted in the 1990 Census.Andover was incorporated as a borough by an act of the New Jersey Legislature on March 25, 1904, from portions of Andover Township. The borough was named for Andover, Hampshire, England.