Hopatcong 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. The area had been called "Brookland" in the 19th century and the lake that was expanded to become Lake Hopatcong had been known as "Great Pond" or "Brookland Pond". During the 1830s, the name of the community had been modified to "Brooklyn", to match the spelling of the city on New York's Long Island. Hopatcong was originally established as the Town of Brooklyn on April 2, 1898, from portions of Byram Township. On March 22, 1901, the Borough of Hopatcong replaced Brooklyn. In 1922, residents of Byram Cove, Northwood, and other areas to the west of the original land area of the borough, voted to leave Byram Township and join Hopatcong, leaving the Borough with its current borders.