The large arionid slug Arion vulgaris is an invasive pest dispersing through large parts of Europe and causing considerable damage in gardens, horticulture and agriculture. It is also possible that this so-called “Iberian slug” has an impact on Norwegian ecosystems, displacing or hybridizing with the native black slug Arion ater. The taxonomy of the large arionids is complex and confusing, encompassing different anatomical forms and colour varieties. The present study integrates, for the first time, coloration, ligula morphology, genital morphometry, nuclear and mitochondrial DNA in order to differentiate the large arionids found in Norway, A. vulgaris, A. ater and A. rufus. These data revealed a clear separation between A. vulgaris and A. ater based on the morphology of the genitalia and mtDNA. However, introgression with the red slug A. rufus was apparent in approximately half of the A. ater specimens analysed, evidenced by ligula morphology, mitochondrial and nuclear DNA. In addition, the mitochondrial COI gene and the nuclear ITS1 gene suggested introgression between A. ater and A. vulgaris. Phylogenetic analyses that included A. vulgaris, A. ater and A. rufus from other parts of Europe, together with A. flagellus, A. fuscus, A. lusitanicus and A. nobrei suggest that A. vulgaris is more closely related to A. ater and A. rufus than to A. lusitanicus. This study confirms the uncertainty of identification based solely on coloration and stresses the importance of integrating several approaches to differentiate these large arionids, allowing a better appreciation of their invasive potential, ecological impact and current distribution.