ABSTRACT Islands harbour unique biodiversity, and depending on their geological origin, they have fundamentally different colonisation histories: continental islands may contain relict faunas shaped by vicariance and sporadic gene flow, whereas oceanic islands must have been colonised across open seas, often through long‐distance dispersal events. Mercuria is a species‐rich genus of freshwater gastropods (33 extant nominal species) widely distributed across the western Palaearctic, including both continental and insular regions, making it a promising candidate for investigating the dispersal modes and colonisation routes of freshwater invertebrates to Atlantic and Mediterranean islands. Yet, species identities, taxonomy and phylogenetic relationships within the genus remain poorly resolved. Here, we present a multilocus phylogeny based on three genes for 216 individuals representing 14 species, with sampling spanning continental populations and insular lineages from the Madeira, Canary, Balearic and Maltese islands, as well as Great Britain. Our results reveal an unrecognised island endemic species in Gran Canaria (Canary Islands), confirm that Madeira harbours the widespread M. tachoensis rather than an endemic species, and provide the first genetic evidence of M. similis in Majorca (Balearic Islands). Biogeographic reconstructions suggest that insular populations originated through multiple passive dispersal events from continental Europe and North Africa, as divergence times (Late Miocene onwards) postdate the island formation. Phylogeographic analyses of widespread species further showed limited dispersal and a single colonisation of Minorca by M. balearica , whereas M. similis and M. tachoensis exhibited broader connectivity. Collectively, these findings reveal complex, lineage‐specific colonisation dynamics in Mercuria , highlighting unrecognised diversity and conservation needs.