Brookside is a historic unincorporated community and census-designated place located within Mendham Township in Morris County, in the U.S. state of New Jersey. It is located approximately 6 mi west of Morristown, the county seat. 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.