The food web positioning of the non-native round goby (Neogobius melanostomus Pallas, 1814) was studied in a new invasive population in Mariehamn, Åland Islands (northern Baltic Sea). The trophic position and isotopic niche space was compared to other benthic-feeding fish species in the same habitat. The trophic position (TP) was estimated based on stable isotope analysis of carbon ( 13 C: 12 C) and nitrogen ( 15 N: 14 N) ratios and compared to that of an established invasive population in Hel, Gulf of Gdansk (southern Baltic Sea). Ontogenetic changes in isotope signatures were evaluated with a regression analysis and compared between the two populations. The round goby positioned as a second-order consumer on the third trophic level among other benthic-feeding fish. It showed a similar trophic position and significant isotopic niche overlap with large perch. The trophic position of round gobies in Mariehamn is significantly higher than in Hel, likely due to different prey items. Furthermore, the ontogenetic patterns differ between the two invasive populations, also suggesting differing resource and habitat availability and levels of intraspecific competition between areas.