Morris Plains 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 6,153, an increase of 621 from the 2010 census count of 5,532, which in turn reflected an increase of 296 from the 5,236 counted in the 2000 census. Morris Plains was incorporated as a borough by an act of the New Jersey Legislature on March 15, 1926, from portions of Hanover Township, based on the results of a referendum held on April 15, 1926.