ABSTRACT Biological invasions introduce non‐native species into natural ecosystems, often reshaping localcommunities and altering trophic interactions. In freshwater environments, such invasions can intensify resource use and threaten native fish diversity. In the Upper Paraná River basin, a hotspot with the highest number of threatened fishes in the Neotropics, invasions can be a significant threat. Understanding how non‐native fish integrate into local food webs, and how their diets overlap or diverge from those of native species, is essential for assessing the risks they pose. This study investigates dietary patterns of the non‐native Poecilia reticulata ( guppy) with those of its closely related native species Phalloceros harpagos in streams of southeast Brazil. We analysed the stomach contents of 77 individuals (31 Poecilia reticulata and 46 Phalloceros harpagos ) sampled, simultaneously, from five streams where the two species coexist. Dietary patterns were evaluated using a combination of ecological approaches, including trophic overlap (i.e., PerMANOVA, nMDS), niche breadth (i.e., Levins' index) and diet composition (i.e., GLMM). Our results indicate that both species rely heavily on dipterans, particularly Chironomidae, exhibiting low niche breadth and high trophic overlap. However, subtle differences in secondary prey consumption suggest a latent dietary overlap, indicating that minor variations in diet may facilitate coexistence despite the strong similarity in primary resources. This dietary partitioning illustrates how reliance on a dominant prey, coupled with opportunistic consumption of less abundant resources, can facilitate coexistence and stabilise trophic interactions in disturbed freshwater streams.