Monmouth County is a county located on the northern part of the Jersey Shore in the U.S. state of New Jersey. The county is part of the New York metropolitan area. As of the 2020 United States census, the county was the state's fifth-most-populous county with a population of 643,615, its highest decennial count ever and an increase of 13,235 from the 2010 census count of 630,380, which in turn reflected an increase of 15,079 from 615,301 at the 2000 census.