Hillsdale is a borough in Bergen County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 10,219, reflecting an increase of 132 (+1.3%) from the 10,087 counted in the 2000 Census, which had in turn increased by 337 (+3.5%) from the 9,750 counted in the 1990 Census.
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 subdivisions to profit from the coming of the Hackensack and New York Railroad (later New Jersey and New York 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 the township of 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.