The presence of the non-indigenous species, the black-pygmy mussel Limnoperna securis, was surveyed for the first time in the Rias of Pontevedra and Arousa, areas adjacent to the first location (Ria of Vigo) of this potential invader in Atlantic waters. Molecular identification of the mussels was conducted by polymerase chain reaction amplification and sequencing of nuclear and mitochondrial genes. This paper describes for the first time the species in the Ria of Pontevedra, confirming that this invader was absent in an intense shellfish farming area of the Ria of Arousa (Galicia, north-western Spain). Field sampling revealed that relatively high concentrations of this mytilid bivalve have colonized some localities of the inner part of the Ria of Pontevedra located in brackish waters. A comparison between population densities, size and substrate preference of L. securis in the Rias of Pontevedra and Vigo was conducted. Dispersion capacity of L. securis is discussed based on molecular detection of larval stages in the stomach contents of the copepod Centropages typicus coupled with physical oceanography of the southern Galician Rias. The invasive role of L. securis is also discussed in the context of the bissus secretion and attachment strength, ecological tolerance of the species, and the recent finding of settlements of this species on numerous colonies of the economically important blue mussel Mytilus galloprovincialis.