Trichocorixa verticalis verticalis, a North American water boatman, is the only alien corixid in European fresh waters. It has rapidly spread, becoming the dominant corixid in and around the Doñana protected area in SW Spain. Its high abundance and similar morphology to native corixids suggest that T. verticalis may impact them through competition for food. Here we used stable isotopes of nitrogen (N) and carbon (C) and Bayesian analytical tools to investigate niche partitioning between T. verticalis and the native water boatman Sigara lateralis and Sigara scripta through a combination of experimental and field data. Species sampled from permanent ponds and laboratory aquaria could be separated based on their isotopic values (δ15N and δ13C). S. lateralis consistently showed higher δ15N values than T. verticalis, suggesting that the invasive species may be feeding at a lower trophic position and relying more on herbivory than its native competitors. This was particularly true for the T. verticalis nymph stage, which showed lower δ15N values compared to adults, indicating ontogenetic dietary shifts. In contrast, native corixids and the invasive species showed similar isotopic compositions in temporary habitats at different stages in the flooding-desiccation cycle, suggesting some degree of niche overlap and a slight reduction in trophic level for S. lateralis when inhabiting ephemeral systems. The combination of experimental and field data can help elucidate trophic interactions during a biological invasion. Stable isotopes provided insights into the trophic ecology of this invasive species and into the mechanisms facilitating co-existence with native species.