Lamont, E. E. (Local Flora Committee, Torrey Botanical Society, The New York Botanical Garden, Bronx, NY 10458) and S. M. Young (New York Natural Heritage Program, 625 Broadway, Albany, NY 12233). Juncus diffusissimus, an addition to the flora of New York, with notes on its recent spread in the United States. J. Torrey Bot. Soc. 132: 635-643. 2005.—The first New York State record for Juncus diffusissimus Buckley (Juncaceae) is reported from Suffolk County, Long Island. An historical account of the species' original range and post-1900 migration in the United States is presented. The nativity status of J. diffusissimus and several other species that have spontaneously migrated into new territories in eastern United States during the past 100 years is discussed.