Among the greatest challenges that Australian flying-foxes (Pteropus spp.) face in the warming Anthropocene are shifts in the intensity and frequency of extreme heat events. Drawing from a suite of behavioural, physiological, demographic, and weather data, we show that such events have dramatic effects on these species, with temperatures exceeding 42oC causing species-, sex- and age class-specific mass mortality at landscape scales. We illustrate this using our data from recent summer heat events, during which more than 60,000 flying-foxes died. In addition, we demonstrate that extreme heat events are already frequent sources of extrinsic mortality for flying-foxes, which is of increasing concern given that temperature extremes are set to escalate further this century. To help land managers, wildlife carers, and other stakeholders respond effectively to these events, we have developed an online flying-fox heat stress forecaster (bit.ly/FF-heat-stress-forecaster). This forecaster makes explicit, up to 72 hours in advance, where and when flying-foxes are most likely to be exposed to heat stress, and thus enables efficient allocation of wildlife management resources towards colonies that are most at risk. Furthermore, our forecaster includes a data portal where citizen scientists can provide urgently needed data about these events. Flying-fox die-offs are particularly conspicuous due to the colonial nature of the species, and it is likely that heat events have similar impacts in wildlife with more solitary and cryptic lifestyles. Thus, flying-foxes provide a rare and valuable opportunity to understand and manage the impacts of extreme heat events on natural systems.