Riverdale is a borough in Morris County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 4,107, an increase of 548 from the 2010 census count of 3,559, which in turn reflected an increase of 1,061 from the 2,498 counted in the 2000 census. Riverdale was incorporated as a borough by an act of the New Jersey Legislature on March 12, 1923, from portions of Pequannock Township, subject to the results of a referendum passed on April 17, 1923.