Keansburg is a borough in Monmouth County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 9,755, a decrease of 350 from the 2010 census count of 10,105, which in turn reflected a decline of 627 from 10,732 in the 2000 census. Keansburg was formed as a borough by an act of the New Jersey Legislature on March 26, 1917, from portions of both Middletown Township and Raritan Township, based on the results of a referendum held on April 17, 1917.