Estuaries, bays and other sheltered coastal areas are frequent environments of marine invasions. These invasive species can have great impacts mainly when they reach high density or if they have new functional attributes. Invasive bivalves are considered ecosystem engineers with this capacity. The invasive mussel Xenostrobus securis, a small brown mussel native to the brackish waters of New Zealand and Australia, has been recently reported in the Ria de Vigo (Galicia, NW Spain). Its distribution overlaps that of the commercial blue mussel Mytilus galloprovincialis along intertidal shores in estuarine areas, i.e. inner part of rias. Here, we carried out a study to assess the patterns of distribution of the two mussel species. In addition, we analysed biotic and abiotic factors that could modulate such patterns. Results showed a gradient of decreasing abundance of X. securis from areas close to the mouth of rivers to the middle part of rias. In contrast, abundances of M. galloprovincialis showed an opposite trend. Salinity was the factor that explained most of variation in abundances, especially in X. securis, although some biotic interactions and factors, such as diversity of the resident community, might be also relevant.