Prospect Park is a borough in Passaic County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 5,865, reflecting an increase of 86 (+1.5%) from the 5,779 counted in the 2000 Census, which had in turn increased by 726 (+14.4%) from the 5,053 counted in the 1990 Census. The borough of Prospect Park was formed by an act of the New Jersey Legislature on March 13, 1901, from portions of the now-defunct Manchester Township. The borough was named for Prospect Park, Brooklyn.