Lake Hopatcong is a census-designated place (CDP) in Jefferson Township, Morris County, New Jersey, United States. It includes communities on the northeast side of Lake Hopatcong, the largest freshwater body in the state. As of the 2020 United States census, the population was 10,232.

The Lake Hopatcong CDP occupies the southwest end of Jefferson Township in western Morris County. It is bordered to the west by the borough of Hopatcong and to the north by Sparta Township, both in Sussex County. It is partially bordered to the south by the borough of Mount Arlington and slightly by the Lower Berkshire Valley CDP.

Lake Hopatcong, the water body, occupies part of the western boundary of the CDP, while the northernmost arm of the lake is entirely within the CDP. Lakefront communities in the CDP include Espanong, Prospect Point, Raccoon Island, Halsey Island, Woodport, and Brady Cove. Inland communities in the CDP include Hurdtown, Tierneys Corner, and Berkshire Valley. Lake Shawnee is in the northeast part of the CDP. New Jersey Route 15 passes through the eastern side of the CDP, leading south 4 miles (6 km) to Interstate 80 in Wharton and north 6 miles (10 km) to Sparta.

Liffy Island (sometimes spelled as "Lify") is located within the CDP and has many walking trails and great views. The island was formed after the creation of the Morris Canal that raised the water by 12 to 14 feet (3.7 to 4.3 m), thus detaching it from the mainland. In the 19th century, the island and surrounding lands were purchased by the Brady Brothers. They sold some of the island's trees as lumber but otherwise left it in its natural state. About 77 acres (31 ha) of the island was bought by Richmond Boy Scout Council Executive Major Walter S. Stewart and Charles McAteer in 1922. They named it "Scout Island" and used it for the Boy Scouts program until 1982. The camp's cook, William "Pete" Peterson, was given lifetime rights to use the island. In the 1970s, William's heath declined which kept him from visiting the island which left it unattended for years. In the 1990s, Jefferson Township, in which Lake Hopatcong CDP is located, bought the island and established it as the Liffy Island Wildlife Preserve. There is a path from Prospect Park and the Prospect Point Road Trailhead that leads to the island. At the highest point is a firepit and sitting area.

According to the U.S. Census Bureau, the Lake Hopatcong CDP has a total area of 9.39 square miles (24.32 km), of which 7.39 square miles (19.14 km) are land and 2.00 square miles (5.18 km), or 21.34%, are water. The lake, with an outlet 2 miles (3 km) to the southwest into the Musconetcong River, is part of the Delaware River watershed.

The community was first listed as a CDP prior to the 2020 census.

Historical population

Census	Pop.	Note	%±
2020	10,232		—
U.S. Decennial Census 2020

People who were born in, residents of, or otherwise closely associated with Lake Hopatcong include:

• Tim Jacobus (born 1959), artist best known for illustrating the covers for nearly one hundred books in R. L. Stine's Goosebumps series
• Jaren Sina (born 1994), professional basketball player, who played for Astoria Bydgoszcz of the Polish Basketball League