Non-indigenous species may have negative impacts on the native fauna in their competition for food and habitat, but they can also introduce non-indigenous parasite species, with sometimes devastating consequences. Co-introduction of parasites should therefore be carefully monitored but this aspect is mostly overlooked. The round goby Neogobius melanostomus and the tubenose goby Proterorhinus semilunaris, both known for their invasiveness, have recently been discovered in Belgium. Here we morphologically and genetically document the co-introduction of the Ponto-Caspian Gyrodactylus proterorhini Ergens, 1967, originally described on tubenose goby in southern Slovakia. Because of their direct life cycle and extraordinary reproductive capacities, gyrodactylid monogenean parasites can readily invade new areas together with the host. Moreover, G. proterorhini has a wide host range, and might therefore represent a threat to other gobiid fishes. The Gyrodactylus parasite found on the Belgian round goby population is probably acquired through secondary infection from local fish, as suggested by molecular phylogenetic analysis.