Next-generation sequencing is increasingly used in conservation biology to resolve complex interactions between species, either diet or gut parasites studies. We applied a recent long metabarcoding method to elucidate the green whip snake’s (Hierophis viridiflavus) prey consumption based on DNA extracted from stomach contents. Illegally introduced in Canton of Vaud (Switzerland), three populations of the green whip snake have strongly developed in two regions, East (Chablais) and North. We suspect that this introduced species is threatening part of the local herpetofauna, especially the Asp viper and the Western green lizard in this region. Consequently, an extermination program has been implemented from 2016 to mitigate Hierophis viridiflavus expansion and its impact arising from its generalist diet. Stomach contents of 94 individuals removed from introduction sites were analysed by long metabarcoding. Our study revealed the consumption of 67 prey belonging to 9 species, primarily small mammals and reptiles. The recurrent presence of two parasitic nematodes was also discovered. Although cannibalistic behaviour could not be highlighted with this approach, a scavenging behaviour was suspected based on the presence of an insect used in forensic entomology (Calliphora vicina). These results confirm the opportunistic feeding behaviour of Hierophis viridiflavus and its ability to predate on threatened species. Although 86.6 % of preys were not listed on the Swiss Red List, the impact on the Asp viper population can be important (up to 20 % of consumed preys) and could partially explain its strong decline.