Biological invasions are one of the greatest threats to biodiversity, economic development, and human health. Therefore an important challenge is to understand the mechanisms and factors that facilitate the spread of invasive species. The Ponto-Caspian polychaete Hypania invalida is one of the invaders that have been colonizing the river systems in Europe since the 1950s. The research aim was to map the distribution of H. invalida in the upper Oder and associated aquatic environments, to assess the contribution of H. invalida to the benthic fauna, to identify environmental factors linked to the occurrence of H. invalida, and to provide genetic markers that can be used to identify H. invalida and monitor its dispersal. H. invalida was found at two sites in the canalized section of the upper Oder and one site at the initial section of the Gliwice Canal. It was found at low abundance and coexisted with other alien macroinvertebrates. H. invalida was not recorded in any smaller river, whether natural, semi-natural, or anthropogenically modified. Apart from the watercourse width, no significant relationships between this polychaete and habitat drivers were found. Genetic analysis showed that the amplified cytochrome c oxidase subunit I and 18S gene fragments showed no sequence variation across all analyzed specimens. Further research is needed to follow the spread of H. invalida in the upper Oder River catchment, as at high densities it can adversely affect inhabited ecosystems.