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Predicting climate change‐mediated environmental effects on organisms is difficult because their direction and strength may depend on multiple ecological factors that vary in nature. This is because the effects of environmental factors often interact. One potentially important factor modifying organisms' thermal performance is their resource level.We experimentally examined the dependence of phenotypic responses to heatwaves on resource/food availability in the freshwater snail, Lymnaea stagnalis. We maintained snails at different food levels (unlimited, reduced [50% of expected consumption], no food), and exposed them to either a heatwave temperature (27°C) or a benign temperature (17°C) for 1 week, followed by a post‐heatwave week (17°C). To estimate snail performance, we quantified their metabolic activity (respiration rate), growth rate, fecundity, and immune activity.All examined traits responded to temperature, and changes in fitness‐related traits (growth rate, fecundity) depended on snail resource level. During the heatwave phase, the benefits of the elevated temperature (enhanced growth rate and fecundity) were higher with unlimited than with limited resources. Additionally, during the post‐heatwave phase, the performance of previously heat‐challenged snails reduced most strongly in resource‐limited snails. The high‐temperature treatment negatively affected snail immune activity even under the high food supply, but immune traits recovered after the heatwave. Interestingly, the phenotypic effects of the high temperature were mainly direct rather than driven by increased metabolic activity.Our results indicate that heatwave responses of L. stagnalis snails are sensitive to their resource level. Such condition dependence may have important ecological and evolutionary implications by: (1) making natural populations in low‐quality habitats the most susceptible to heatwaves; and (2) altering climate change‐mediated evolution by modifying the expression of phenotypic variation in thermal performance. By demonstrating condition‐dependent heatwave responses, our study highlights the importance of examining the interactive effects of climate change‐mediated environmental alterations and ecological factors that vary in nature. [ABSTRACT FROM AUTHOR]