The introduction and naturalization of alien fish species in the basins of the largest rivers of the Ponto-Caspian Basin—Dnieper, Don, and Volga—are reviewed. Over the last 50 years, over 50 new fish species have appeared in the basins of these rivers; 25 of them can be considered naturalized. The invasive species are classified according to their ecological and taxonomic categories and modes of life. The principal vectors of the invasions are described. Southern alien species are doubtlessly prevalent in species number; among them, species of the Ponto-Caspian Basin are prevalent. The greatest ecological success is found in species prone to autoexpansion. The dynamics of the introduction of new species into natural ecosystems is analyzed. The invasion process in the studied river basins is found to accelerate both in number of newly discovered species and in naturalization rate.