Hillsdale is a borough in the northern portion of Bergen County, in the U.S. state of New Jersey, within the New York City metropolitan area. As of the 2020 United States census, the borough's population was 10,143, a decrease of 76 from the 2010 census count of 10,219, which in turn reflected an increase of 132 from the 10,087 counted in the 2000 census. The populated area today known as Hillsdale took form in the mid-to-late 19th century as land speculators, led by David P.