Abstract <p>The paper presents the results of 2009–2023 studies of ecological characteristics of the amphipod <i>Pontogammarus robustoides</i> (Sars, 1894) in water bodies of the Volga, Kama, and Don rivers. It is shown that <i>P. robustoides</i> is distributed throughout the Volga and Kama cascade of reservoirs, is registered in the Voronezh Reservoir, and has also dispersed into tributaries of the Kuybyshev, Saratov, and Volgograd reservoirs. The frequency of occurrence of <i>P. robustoides</i> in reservoirs averages 10–20%; in rivers, it occurs sporadically, mainly in their lower reaches. The average biomass of <i>P. robustoides</i> in reservoirs did not exceed 0.2 g/m², in some places, the species forms mass accumulations up to 10.3 g/m². It was found that, in the conditions of the Volga and Kama reservoirs, the species demonstrates high ecological plasticity to master different habitats: sands and silts with plant remains and clay, thickets of higher aquatic plants, depressions of objects submerged in water. The paper considers the impact of natural environmental factors (bottom water temperature, pH, depth, transparency, dissolved oxygen) on the occurrence of <i>P. robustoides</i>. It is also shown that in reservoirs the species is most often found with some (including alien) species of gastropod mollusks, as well as omnivorous smaller species of amphipods and mysids. In the surveyed rivers, the three largest amphipod species <i>Dikerogammarus villosus</i> (Sowinsky, 1894), <i>D. haemobaphes</i> (Eichwald, 1841) and <i>P. robustoides</i> often occurred together, and their interaction is an example of conjugate invasion.</p>