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The present study offers a comprehensive analysis of changes in the abundance and diet composition of juvenile flounder ( Platichthys flesus ) and turbot ( Psetta maxima ), along with other dominant coastal fish species, before and after the establishment of the alien round goby off an exposed stretch of coast in the eastern Baltic Sea. In the study area, the round goby ( Neogobius melanostomus ) was recorded for the first time in 2009. After a few years of low abundance, a sharp increase in the population occurred. After the round goby invasion, flatfish juveniles exhibited an increased diet overlap with other species and had a lower feeding success, reflecting an increase in resource competition. For juvenile turbot, the increase was mainly caused by the round goby, while for flounder it was due to both the round goby and the lesser sandeel ( Ammodytes tobianus ). Juvenile turbot, whose dominant food item before the round goby establishment had been mysids, shifted their diet towards Crangon crangon , reflecting a decrease in mysid abundance by three orders of magnitude and a concurrent doubling in C. crangon abundance in the habitat. At the same time a significant decrease in turbot recruitment was observed. Juvenile flounder had the widest food spectrum of the studied species. When the availability of the primary food item, Bathyporeia pilosa , decreased, flounder juveniles adapted by increasing the share of zooplankton in their diets. No changes in flounder feeding success and recruitment were observed. However, the recruitment estimates of flounder and turbot show an increasing co-variation after the round goby invasion, suggesting that recruitment of the species may currently be regulated by processes in the common nursery habitat.