The dark false mussel Mytilopsis leucophaeata is an invasive bivalve known from two locations in Rio de Janeiro (Brazil): Lagoa Rodrigo de Freitas and Lagoa de Marapendi. For a population biology study of this species, monthly collections were carried out at Lagoa Rodrigo de Freitas over a period of 2 y (March 2016–March 2018). High densities were registered in both studied years: 84,560 (±22,066) ind. m^–2 in the first year and 65,848 (±7,830) in the second year. Growth was lower for the first year (K = 0.4 for year 1 and K = 0.8 for year 2), whereas seasonal oscillation (C = 0.6) was of the same intensity in both years. The period of lower growth differed for the years, occurring in June (WP = 0.5) in the first year and in August (WP = 0.7) in the second year. The maximum and asymptotic lengths, longevity, secondary production, biomass, and turnover rate were higher for the first year; on the other hand, mortality was higher at the second year. Density of the invasive mussel was significantly and positively related with salinity (r² = 0.5547; P < 0.0001). In a general way, the second year of study seems to be a more adverse period for the population of the invasive mussel with reduced density, body sizes, life expectancy, secondary production, biomass, and higher mortality. Among environmental factors, salinity seems to exert the strongest influence on the regulation of population attributes.