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 (-4.7%) from the 15,888 counted in the 2000 Census, which had in turn increased by 302 (+1.9%) from the 15,586 counted in the 1990 Census.
The town of Hopatcong has a rich history, given its relationship with New York City. It borders Lake Hopatcong, a partially man-made lake that is now a source of much recreation and desirable real estate, and is the biggest lake in New Jersey. The community, 40 miles (64 km) west of New York City, began as a summer getaway for the wealthy in NYC who primarily sought access to the lake. An amusement park, called "Bertrand's Island", sprang up and was accessible via the lake, trolley or by car through Mount Arlington. The construction of Interstate 80, a highway that stretches from Teaneck, New Jersey, all the way across the country to San Francisco, California, triggered rapid growth in New York City's suburbs and led to Hopatcong becoming a permanent residential community.
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.