AbstractChanges in temperature associated with climate change can alter species' distributions, drive adaptive evolution, and in some cases cause extinction. Research has tended to focus on the direct effects of temperature, but changes in temperature can also have indirect effects on populations and species. Here, we test whether temperature can indirectly affect the fitness of Drosophila santomea and Drosophila yakuba by altering the nature of interspecific competition. We show that when raised in isolation, both D. santomea and D. yakuba display similar variation in relative fitness across temperatures of 18°, 22°, and 25°C. However, D. santomea has higher fitness than D. yakuba when experiencing interspecific competition at 18°C, while the inverse is true at 25°C. Patterns of fitness across thermal and competitive environments therefore indicate that the outcome of interspecific competition varies with temperature. We then use a coexistence experiment to show that D. santomea is rapidly (within eight generations) extirpated when maintained with D. yakuba at 25°C. By contrast, D. santomea remains as (or more) abundant than D. yakuba over the course of ∼10 generations when maintained at 18°C. Our results provide an example of how the thermal environment can affect interspecific competition and suggest that some species may become more prone to extinction under scenarios of climate change through indirect effects of the thermal environment on competitive advantages between species.