Riella affinis Howe & Underwood is a rare species that is thus far known from eight populations worldwide. Only half of these were recorded in the last 50 years and none were recollected since first reported. Study of a Spanish population of R. cossoniana Trab., recorded in 2002 from Las Lomas, Malaga province, revealed that it belongs to R. affinis. The correct identity of the populations, which had not been available for study, was already suggested by reproductive and ecological characteristics of the plants mentioned in the original publication. The three main lagoons from the Las Lomas endorheic system, where the plants had been recorded, were sampled and living plants of R. affinis were found in Viso lagoon whereas culture of sediments yielded plants of this species from Viso and Caja lagoons. Riella affinis is new to continental Europe and the new Spanish populations are described and illustrated in detail. Riella cossoniana is excluded from Malaga province. Riella affinis is classified as critically endangered (CR) globally and for Spain; the new Iberian populations should be targeted for conservation. An identification key to the currently recognised European species of Riella Mont. is included.