Biological invasions are a major cause of biodiversity loss and, coupled with climate change, will likely have detrimental impacts for native species and the functioning of ecosystems. To mitigate such impacts, it is important to elucidate the behavioural mechanisms underpinning interactions between invasive and native species. Here we examined how competitive interactions between invasive and native species are modified under conditions of near-future elevated water temperature using freshwater crayfish as a model system. Contest experiments between the native Euastacus spinifer and invasive Cherax destructor revealed that the competitive advantage of E. spinifer at current maximum temperatures (22 °C) was reversed at elevated near-future temperatures (26 °C), after controlling for relative body size. In addition, the native crayfish spent twice as long motionless at 26 °C than C. destructor, consistent with physiological challenges underpinning this competitive reversal. Most alarmingly, E. spinifer experienced significant mortality after fighting C. destructor, particularly at 26 °C. Mortality usually ensued two days post-contest even when E. spinifer had won. Mortality was rare when fighting conspecifics. Together, these results suggest that while E. spinifer is more likely to win contests under current conditions, it could suffer considerable impacts if climate change and the spread of invasive C. destructor continue unabated.