Robust estimates of marine species vulnerability to ongoing climate change require realistic stressor experiments. Here, we subjected an important coastal predator, the sea star Asterias rubens, to projected warming and ocean acidification over an annual seasonal cycle. Warming and, less so, acidification, had strongly season-specific impacts on animal energy budgets. Specifically, simulated future summer temperatures caused >95% sea star mortality, reduced feeding rate and body mass loss. Additional acute experiments demonstrated that respiratory oxygen flux was preferentially directed to support high summer metabolism at the expense of feeding-related processes. Using 15 years of field temperature data and end of century warming projections, we estimate that potentially lethal summer heat waves will occur in 20% of future years. Our study demonstrates the importance of assessing stress responses along seasonal thermal cycles and the high selective force that future summer heat waves likely can exert on coastal marine animal populations.