We report the occurrence of alien grass snakes (Natrix natrix ssp.) in Western Switzerland (Lausanne), at a locality where striped grass snakes, presumably from Croatia (N. n. persa), escaped from outdoor enclosures of a reptile park in the 1970s, within the natural range of the threatened barred grass snake (N. n. helvetica). Three gravid females were captured at the introduction site in 2016: two exhibited colour patterns typical of native N. n. helvetica and the third had an intermediate phenotype. Yet, their neonates featured diverse colour patterns, several typical to N. n. persa. Genetic analyses identified two distinct mitochondrial alien lineages: one specific to N. n. persa/cypriaca and one specific to south-Alpine N. n. helvetica (genetically unrelated to native Western Swiss N. n. helvetica). These results suggest that alien grass snakes of multiple origins have established sustainable populations at the introduction site for the past 40 years, and potentially hybridized and introgressed with the local taxa. Hence, it alarms on the captive breeding practices of potentially invasive reptiles in animal parks, and adds to a long list of uncontrolled herps’ introductions, a major threat to biodiversity in Western Europe.