Andover is a borough in Sussex County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 595, a decrease of 11 from the 2010 census count of 606, which in turn reflected a decline of 52 from the 658 counted in the 2000 census. Andover was incorporated as a borough by an act of the New Jersey Legislature on March 25, 1904, from portions of Andover Township.