Most benthic invertebrates and plant species that survive in the brackish Baltic Sea are characterized by populations in which growth is reduced and genetic diversity is low, indicating that these species live under marginal conditions. In this study, we tested for a salinity-related growth rate reduction in 21 European Macoma balthica populations. Additionally, we transplanted clams into hyposaline environments, along the Baltic Sea salinity gradient, and compared survival, growth and condition with control groups. All transplants into hypoosmotic environments were successful. Despite the osmotic shock, the results showed no reduced growth rates in transplanted individuals. Only the clams that had been transplanted into the extremely oligohaline Gulf of Bothnia (0.5-3.0 PSU) exhibited reduced growth rates in comparison to the control group. From this study we conclude that M. balthica has an exceptional capacity to acclimatize to extremely brackish conditions, which may explain its abundance and diversity in the Baltic Sea.