Cliffside Park is a borough in Bergen County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 25,693, an increase of 2,099 from the 2010 census count of 23,594, which in turn reflected an increase of 587 from the 23,007 counted in the 2000 census. Cliffside Park was formed based on the results of a referendum held on January 15, 1895, from portions of Ridgefield Township.