An extensive field survey was conducted to evaluate the spreading of the invasive estuarine bivalve Mytilopsis leucophaeata into the metropolitan region of Rio de Janeiro (Brazil), in which the single invaded site so far recorded is Rodrigo de Freitas Lagoon. A novel record is made to the Marapendi coastal lagoon, where M. leucophaeata mainly colonizes mangrove roots. A reef composed of druses above the unconsolidated substratum is recorded for the first time for this species. The fauna associated to agglomerates of M. leucophaeata was composed of eight taxa: polychaetes Alitta succinea and Polydora sp., amphipods Melita mangrovi and Elasmopus sp., barnacles Amphibalanus eburneus and A. improvisus, the crab Eurypanopeus cf. dissimilis and the gastropod Heleobia spp. Based on COI sequences, the haplotype diversity of M. leucophaeata invasion in Marapendi Lagoon is high, like that previously recorded in Rodrigo de Freitas Lagoon. The expansion of M. leucophaeata demands a continuous investigation, including the knowledge of its impacts on the biota.