is a borough in Union County, New Jersey, United States. As of the United States 2000 Census, the borough population was 6,602.
Mountainside was incorporated as a borough on September 25, 1895, from portions of Westfield Township, based on the results of a referendum held the previous day.
magazine ranked Mountainside as its 16th best place to live in its 2008 rankings of the "Best Places To Live" in New Jersey.
The borough is bordered to the north by Summit, to the east by Springfield Township, to the south by Westfield, and to the west by Berkeley Heights and Scotch Plains.
According to the United States Census Bureau, the borough has a total area of 4.1 square miles (10.5 km