Lemna valdiviana Phil. and L. minuta Kunth are native species of the Americas. At present L. minuta colonizes in Europa and Asia, where it is often considered an invasive alien species. Recent floristic surveys in the Agro Pontino area (Lazio region, central Italy) allowed to find a population referring to L. valdiviana, supported by the recent gathering in Sardegna region. With the aim to identify correctly the two species, we carried out a morphometric study based on measurements of both qualitative and quantitative characters, made on herbarium and living specimens collected from America and Europe. The univariate and multivariate analyses clearly support the attribution of the population from Lazio to L. valdiviana. This gathering is the first record of this species for the Italian peninsula. Moreover, the checking of Herbaria allowed us to find the older specimen of L. minuta collected in Europe (around 1877). At present L. valdiviana is to be considered as a casual alien species in Agro Pontino, but the rapid colonization of the aquatic environments by Lemna species by means of vegetative reproduction allows us to recognize it as a potential invasive species.