Passaic County is a county in the U.S. state of New Jersey that is part of the New York metropolitan area. As of the 2020 United States census, the county was the state's eighth-most-populous county, with a population of 524,118, its highest decennial count ever and an increase of 22,892 (+4.6%) from the 2010 census count of 501,226, which in turn reflected an increase of 12,177 (+2.5%) from the 489,049 counted in the 2000 census. The most populous place in Passaic County is Paterson, with 159,732 residents at the 2020 Census, more than 30% of the county's population, while West Milford covered 80.32 square miles (208.0 km2), the largest total area of any municipality and more than 40% of the county's area. The county is part of the North Jersey region of the state.

Paterson is the largest city in and the county seat of Passaic County, in the U.S. state of New Jersey. As of the 2020 United States census, the city was the state's third-most-populous municipality, with a population of 159,732.