Field surveys in eastern North America confirm the naturalization of Glossostigma plants at 19 localities in four states: Connecticut, New Jersey, Pennsylvania, and Rhode Island. DNA sequence analysis of individuals from 14 sampled populations identifies these nonindigenous plants as Glossostigma cleistanthum, a species native to Australia and New Zealand. These results correct prior misidentifications of North American plants as G. diandrum. The earliest North American record of G. cleistanthum (1992) is from a Ramsar tidal wetland in Connecticut. Morphological analyses demonstrate that G. cleistanthum differs from G. diandrum by its longer leaves and ability to produce both cleistogamous and chasmogamous flowers in response to ecological conditions. Glossostigma cleistanthum has a high reproductive potential and spreads rapidly within and between both artificial and natural habitats. A survey of more than 100 lakes indicated that G. cleistanthum occurs most often in waters with high clarity and low pH, alkalinity, conductivity, and phosphorous. Because of its affinity for oligotrophic conditions, this species is a particular threat to pristine natural aquatic communities, which often contain imperiled plants.