Monitoring terrestrial mammalian biodiversity remains challenging due to the limitations of conventional survey methods, including high labour requirements and detection bias. This study investigated how an extreme rainfall event, Typhoon Khanun (August 2023), influences the detection of terrestrial mammalian environmental DNA (eDNA) in a stream ecosystem. Samples were collected during the typhoon-induced runoff and again after hydrological conditions returned to baseline. Mammalian eDNA was successfully detected only during the rainfall event, whereas no mammalian DNA was detected in post-event samples. Metabarcoding analysis, targeting mitochondrial 12S rDNA, identified ten mammalian species from stormwater runoff, including both wild and anthropogenic taxa, while conventional surveys detected only two species at the same sites. These findings demonstrate that typhoon-driven runoff can transiently mobilise and concentrate terrestrial mammalian eDNA into stream systems, substantially enhancing detection probability. However, the detection of non-resident and anthropogenic species highlights the need for careful interpretation of runoff-derived eDNA signals. Overall, this study shows that extreme rainfall events can act as natural sampling windows that integrate catchment-scale biodiversity signals, providing a complementary approach for terrestrial mammal monitoring.