A newly established population of the fouling polychaete Spirobranchus cf. tetraceros is reported from the western Mediterranean (Valencia Port). Despite previous intensive surveys, this is the first record for the taxon in the Iberian Peninsula. Molecular analyses revealed that S. cf. tetraceros from Valencia are genetically identical to specimens from Heraklion, Crete, but different from those collected in the Red Sea and S. tetraceros sensu stricto from the type locality in Australia. Mediterranean and Red Sea S. cf. tetraceros form a well-supported monophyletic clade but are clearly distinct from New South Wales specimens of S. tetraceros. Our new molecular evidence supports the hypothesis that S. tetraceros is not a global invader of Australian origin but rather a large species complex in need of a comprehensive worldwide revision. These results highlight the importance of integrative taxonomic research for species with reported global distributions because these taxa may include cryptic invaders. An illustrated morphological account of the Valencia and Heraklion specimens and a taxonomic key for Spirobranchus species in the Mediterranean Sea are provided.