As a foundation species in shallow-water ecosystems, the mussel Mytilus edulis exposed to abiotic and biotic stressors. In particular, heat stress can have detrimental effects on mussel performance and biotic interactions with parasites may exacerbate those effects. This study explores the metabolic responses of mussels to infections with the trematode Renicola roscovita, under mild and transient heat exposures. Using controlled laboratory infections, a first experiment investigated the responses of infected and uninfected small mussels (2 cm), to a 24-hour exposure to a mild temperature followed by an acute heat ramp and subsequent cooling. The results indicated that trematode infections reduced mussel filtration by, on average, 13%, and that infections exacerbated the impact of the transient heat exposure, as indicated by a larger difference (33%) between infected and uninfected mussels in the post-heat recovery phase. However, these differences were statistically not significant, owing to generally large variation among mussels and low sample sizes. In a second experiment, we further investigated how mussel size (2 and 4 cm) affected infection impacts on mussel performance under constant exposure to a mild temperature. We found that infections reduced mussel filtration at similar rates (11% and 12% for small and large mussels, respectively) compared to the first experiment, albeit statistically significant only for large mussels. In both experiments, no discernible impact on the mussel respiration rate was found. Interestingly, we found positive relationships between mussel filtration capacity and infection intensity, suggesting that mussel phenotypes with generally higher filtration capacity may be more prone to infections. Overall, our results suggest that R. roscovita metacercariae infections can lower the ability of hosts to sustain optimal energy supply, which in turn may exacerbate the negative effects of heat stress. The role of mussel phenotypic variation in filtration capacity in driving infection levels and subsequent effects warrants further investigation.