Hillsdale is a borough in Bergen County, New Jersey, United States. As of the United States 2000 Census, the borough population was 10,087. The populated area today known as Hillsdale took form in the mid-to-late 19th century as land speculators, led by David P. Patterson, developed subdivisons to profit from the coming of the Hackensack and New York Connecting Railroad. The area was incorporated as Hillsdale Township on March 25, 1898, from portions of Washington Township, which had, in turn, been set off from Harrington Township in 1840. Portions of the township were taken on April 30, 1906, to create River Vale. Hillsdale was reincorporated as a borough by an Act of the New Jersey Legislature on March 2, 1923, based on the results of a referendum held on April 24, 1923.