Juncus dichotomus, a native of the Americas, is reported for the first time from Europe. It is more or less widely naturalised in northwestern Italy (provinces of Biella, Novara, Torino and Vercelli) and probably largely overlooked elsewhere as a result of confusion with J. tenuis. Diagnostic features of both species (and other members of the J. tenuis group currently found in Europe, viz. J. anthelatus and J. dudleyi) are critically assessed and an identification key is presented. Taxonomy and nomenclature of J. dichotomus are discussed, especially with regard to the status of J. platyphyllus, and notes on its present distribution in Italy and its ecology are provided. Like J. tenuis, it often grows in valuable natural and seminatural habitats and locally seems to behave like an invasive environmental weed.