During 2010-2013 populations of an unfamiliar species of Succineidae were found to be established in southern Spain (near Málaga), north-eastern Spain (provinces of Barcelona and Tarragona) and west-central Portugal (Leiria and Porto de Mós). The shells, mantle coloration and genital anatomy are described. It is demonstrated that they do not resemble any European species of the family. However, characters of the distal genital anatomy match those of Succinea subgenus Calcisuccinea Pilsbry, 1948, which is native in North and Central America. Nevertheless, there are clear differences from the taxa known anatomically which are currently placed in this subgenus, so it is suggested that the Iberian specimens may be referable to one of the American taxa of Succineidae for which the anatomy is currently unknown and the species level taxonomy unresolved. The anthropogenically disturbed habitats at the Portuguese and some Spanish sites were drier than those occupied by any of the native Succineidae of western Europe. It remains to be seen whether this taxon will become a pest in Europe, where it is probably now too widespread to be eradicated.