Hillside is a township in Union County, New Jersey. As of the 2010 United States Census, the township's population was 21,404, reflecting a decline of 343 from the 21,747 counted in the 2000 Census, which had in turn increased by 703 from the 21,044 counted in the 1990 Census.
Hillside was incorporated as a township on April 3, 1913, from portions of Union Township, based on the results of a referendum held on April 29, 1913. The township was named for the surrounding hills.
The township is split between area codes 908 and 973.