Hillside is a township in Union County, in the U.S. state of New Jersey. As of the 2020 United States census, the township's population was 22,456, an increase of 1,052 (+4.9%) from the 2010 census count of 21,404, which in turn reflected a decline of 343 (−1.6%) from the 21,747 counted in the 2000 census.

Union County is a county in the northern part of the U.S. state of New Jersey. As of the 2020 United States census, the county was the state's seventh-most-populous county with a population of 575,345, its highest decennial count ever and an increase of 38,846 (+7.2%) from the 2010 census count of 536,499. Its county seat is Elizabeth, which is also the most populous municipality in the county, with a 2020 census population of 137,298, and the largest by area, covering 13.46 square miles (34.9 km2). The county serves as a transition point between the Central Jersey and North Jersey regions of the state.