Brookside (once called Waterstreet) is a historic unincorporated community within Mendham Township in Morris County, New Jersey, United States. It is located approximately 6 mi (9.7 km) west of Morristown. It is close to the Washington Valley Historic District in neighboring Morris Township. In the Forbes magazine 2009 ranking of the Most Expensive ZIP Codes in the United States, Brookside was ranked as the 10th most expensive in the country, with its median home sale price of $3,121,115.