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.

Elizabeth is a city in and the county seat of Union County, in the U.S. state of New Jersey. As of the 2020 United States census, the city retained its ranking as the state's fourth-most-populous city behind neighboring Newark, Jersey City, and Paterson, with a population of 137,298, an increase of 12,329 (+9.9%) from the 2010 census count of 124,969, which in turn reflected an increase of 4,401 (3.7%) from the 120,568 counted in the 2000 census.