Simple SummaryReports of invasive species have increased dramatically in recent decades, raising public awareness. While human activities favor their spread, global warming is believed to promote the establishment of alien species and pose a major threat to native ecosystems. The freshwater jellyfishCraspedacustais an example of a globally successful spread, attracting public attention with its short-lived, eye-catching medusa blooms. Two life stages of this jellyfish are predators: the benthic polyps and the planktonic medusae. Both must meet the requirements in their new food webs to become established. We compared their niches with those of presumed native competitors and found a strong overlap for the medusa stage, suggesting high competition with native zooplanktonic predators and young fish. In contrast, the polyps of invasiveCraspedacustaand nativeHydradiffered in their niches, possibly favoring a long-term invasion success ofCraspedacusta.AbstractWhen species spread into new regions, competition with native species and predatory–prey relationships play a major role in whether the new species can successfully establish itself in the recipient food web and become invasive. In aquatic habitats, species with a metagenetic life cycle, such as the freshwater jellyfishCraspedacustawith benthic polyps and planktonic medusae, have to meet the requirements of two distinct life stages occurring in two habitats with different food webs. Here, we examined the trophic position of both life stages, known to be predatory, and compared their niches with those of putative native competitors using stable isotope analysis. We found that δ13C and δ15N signatures of medusae overlapped with those of co-occurringChaoboruslarvae and juvenile fish (Rutilus rutilus) in a well-studied lake, implying high competition with these native predators. The comparison of δ15N signatures ofHydraandCraspedacustapolyps in four additional lakes revealed their similar trophic position, matching their predatory lifestyle. However, their δ13C signatures differed not only across all four of the lakes studied but also within one lake over time, suggesting a preference for pelagic or benthic food sources. We conclude that invasive and native polyps differ in their niches due to different food spectra, which favors the invasion success ofCraspedacusta.