Sphenoclea zeylanicais described and its phenology, habitat, and distribution documented in North America, based upon data from voucher specimens and original field observations. The species, an exotic native to Asia, has been in the United States since the mid-1800s, with the earliest records from Louisiana where it was most likely introduced as a contaminant of rice seed. Its distribution and spread are linked strongly with cultivation of rice, and we document its dispersal in the United States over the past 150 years with the postbellum expansion of rice agriculture in the Mississippi River Valley. Photographs and scanning electron micrographs illustrate structural details of inflorescence, flower, fruit, and seed.