Neptune City is a borough in Monmouth County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 4,626, a decrease of 244 from the 2010 census count of 4,869, which in turn reflected a decline of 349 from the 5,218 counted in the 2000 census. The Borough of Neptune City was incorporated on October 4, 1881, based on a referendum held on March 19, 1881.