Cliffside Park is a borough in Bergen County, New Jersey, United States. As of the 2010 United States Census, the borough population was 23,594. Cliffside Park was formed based on the results of a referendum held on January 15, 1895, from portions of Ridgefield Township at the start of the second year of the Boroughitis phenomenon then sweeping through Bergen County. U.S. Senator Frank Lautenberg currently resides in the borough.