Brielle is a borough located in southern Monmouth County, in the U.S. state of New Jersey, along the Manasquan River. As of the 2020 United States census, the borough's population was 4,982, its highest decennial count ever and an increase of 208 from the 2010 census count of 4,774, which in turn reflected a decline of 119 from the 4,893 counted in the 2000 census.