Earlier morpho-anatomical work showed that a non-native Ulva species is widely distributed off the northwestern Iberian Peninsula (NWIP) since the early 1990s. However, insufficient sequence data provided at the time of the earlier study led to suggestions that more genetic evidence might be required to confirm this record. Here, we present this evidence in sequence data for the nuclear-encoded ITS1 and the plastid-encoded rbcL. We confirm the occurrence of an alien distromatic Ulva in several locations along the NWIP. Additionally, its presence in the Spanish Mediterranean (>1000 km away) is reported here for the first time. Species identification yielded notably coherent results between nuclear-encoded and plastid-encoded sequences, revealing that Spanish plants are conspecific with collections of Ulva pertusa and Ulva australis from a range of Pacific locations. The probable synonymy between U. australis and U. pertusa is consistent with their morphological similarity and suggests that U. australis is the correct name for the non-native green macroalga. ITS1 sequences reveal a common source for the various sites studied in the NWIP, possibly linked to shellfish transfers into the region. However, non-native populations in southern Spain must derive from an independent introduction event. This is the first time that molecular data have been used to demonstrate multiple introductions of U. australis to European waters.