Mass mortality is often observed in cultured oysters during the period following spawning in the summer season. In order to examine the underlying causes leading to this phenomenon, thermotolerance of the Pacific oyster, Crassostrea gigas, was assessed using pre- and post-spawning oysters that were sequentially treated with sublethal (37 °C) and lethal heat shocks (44 °C). The effects were examined on a range of immune and metabolic parameters, in addition to mortality rate. A preventative 37 °C significantly reduced oyster mortality after exposure to a second heat shock of 44 °C, but in post-spawning oysters mortality remained at 80%, compared to less than 10% in pre-spawning oysters. Levels of the 72 kDa and 69 kDa heat shock proteins were low in the gill tissue from post-spawning oysters stimulated by heat shock, indicating spawning reduced heat shock protein synthesis. The post-spawning oysters had depleted glycogen stores in the mantle tissue and reduced adenylate energy charge (AEC) after heat shock, indicative of lower energy for metabolic activity. A cumulative effect of spawning and heat shock was observed on the immunocompetence of oysters, demonstrated by reduced hemocyte phagocytosis and hemolymph antimicrobial activity. These results support the hypothesis that the energy expended during reproduction compromises the thermotolerance and immune status of oysters, leaving them easily subject to mortality if heat stress occurs in post-spawning stage. This study improves our understanding of oyster summer mortality and has implications for the long-term persistence of molluscs under the influence of global warming.