Abstract. Abstract: Aim Non-native fish species have been closely related to serious damage to aquatic biodiversity due to their negative effects on native fauna. We aim to report the first and alarming occurrence of the non-native catfish “jaú” Zungaro jahu in the Iguaçu River basin above the Iguaçu Falls, a high-endemism Neotropical ecoregion for fish. Methods Fish samplings were taken with a bottom gill net in the Segredo Reservoir, where they were anesthetized in benzocaine hydrochloride and fixed in formaldehyde in the field, later identified in the laboratory and housed at the Museu de História Natural do Capão da Imbuia (MHNCI). Results Our recent samplings in the Iguaçu River basin recorded one non-native individual of the catfish “jaú” Zungaro jahu in the Segredo Reservoir. Conclusions We mainly intend to alarm about the risk of expansion of the occurrence of this non-native species throughout the cascading reservoirs in the Iguaçu River where it could potentially develop serious damage to the trophic chain, predation of small-sized endemic species, and competition, especially in the sites where the endemic and endangered pimelodid “surubim-do-iguaçu” (Steindachneridion melanodermatum) maintains viable populations, currently restricted to the Lower Iguaçu River basin exhibiting recent signs of a population bottleneck.