Hillside is a township in Union County, New Jersey, United States. As of the 2010 United States Census, the township's population was 21,404, reflecting a decline of 343 (-1.6%) from the 21,747 counted in the 2000 Census, which had in turn increased by 703 (+3.3%) 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.
Hillside was created from parcels of land carved out of neighboring Newark, Elizabeth, and Union. It originally contained the farms of Woodruff, Conant and Saybrook. Local streets still bear their names.
Hillside was incorporated shortly after the appearance of Halley's Comet in 1910, and for that reason, the team nickname of Hillside High School is the "Comets." Several local businesses take the name "Comet" for the same reason.
The Hillside Historical Society was established in the 1980s in the Woodruff home on Conant Street, perhaps the township's oldest. The Woodruff House and Eaton Store Museum is operated and maintained by the Hillside Historical Society. Purchased by the Society in 1978, the house has been faithfully restored to its original grandeur. The Woodruff House spans three centuries in one structure, including the original 1735 building, the 1790 addition, the 1890s kitchen and the 1900s store.