Union County is a county in the U.S. state of New Jersey. As of the 2017 Census estimate, the county’s population was 563,892, making it the seventh-most populous of the state’s 21 counties, an increase of 5.1% from the 2010 United States Census, when its population was enumerated at 536,499, in turn an increase of 13,958 (2.7%) from the 522,541 enumerated in the 2000 Census. In 2010, Union County slipped to the seventh-most populous county in the state, having been surpassed by Ocean County. Union County is part of the New York metropolitan area. Its county seat is Elizabeth.

In 2015, the county had a per capita personal income of $60,089, the seventh-highest in New Jersey and ranked 152nd of 3,113 counties in the United States.  The Bureau of Economic Analysis ranked the county as having the 119th-highest per capita income of all 3,113 counties in the United States (and the eighth-highest in New Jersey) in 2009. A study by Forbes.com determined that Union County pays the second-highest property taxes of all U.S. counties, based on 2007 data.

With a population density of 4,955 people per square mile (water excluded), Union County was the 15th-most densely populated county in America as of the 2010 Census, and third-densest in New Jersey, behind Hudson County (ranked 6th nationwide at 9,754 per square mile) and Essex County (ranked 11th at 6,126).