Union City is a city in Hudson County, New Jersey, United States. According to the 2010 United States Census the city had a total population of 66,455, reflecting a decline of 633 (-0.9%) from the 67,088 counted in the 2000 Census, which had in turn increased by 9,076 (+15.6%) from the 58,012 counted in the 1990 Census. As of the 2010 Census it was the most densely populated city in the United States, with a density of 51,810.1 per square mile.