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Monmouth County (/ˈmɒnməθ/) is a county located in the central portion of the U.S. state of New Jersey. It is bordered to its west by Mercer and Middlesex Counties, to its south by Ocean County, to its east by the Atlantic Ocean, and to its north by the Raritan Bay (sharing a border with the boroughs of Staten Island, Brooklyn, and Queens in New York City, across it). Monmouth County's geographic area comprises 30% water. The county is part of the Jersey Shore region of the state.[6] It has also been categorized under the Central Jersey region,[7] though it was not among the four counties explicitly listed as included in Central Jersey as part of legislation signed by Governor Phil Murphy into law in 2023.[8]

As of the 2020 United States census, the county was the state's fifth-most-populous county[9] with a population of 643,615,[3][4] its highest decennial count ever and an increase of 13,235 (+2.1%) from the 2010 census count of 630,380,[10] which in turn reflected an increase of 15,079 from 615,301 at the 2000 census.[11] As of 2010, the county fell to the fifth-most populous county in the state, having been surpassed by Hudson County.[12][13] Monmouth's county seat is Freehold Borough,[1] though many county offices are also located in neighboring Freehold Township.[14] The most populous place was Middletown Township, with 67,106 residents at the time of the 2020 Census,[4] while Howell Township covered 61.21 square miles (158.5 km2), the largest total area of any municipality.[13]