The effect of thermal stress during reproduction was experimentally evaluated in the oyster Crassostrea gigas, a temperate species, and in the tropical oyster Crassostrea corteziensis. The temperature was gradually increased (1 °C day−1) from 20 °C to 34 °C for two weeks. As expected, C. gigas was the species most affected by heat stress, with the highest mortality rate (P < 0.05) starting at 28 °C, while mortality in C. corteziensis was significant only at 34 °C. The reproductive effort at higher temperatures was reflected in C. gigas as the highest index of mature oocytes and the largest rate of atresic and degenerated oocytes. C. corteziensis showed significant increases in the proliferation of early-developing oocytes at maximum temperatures. Lipid peroxidation and lipofuscin accumulation significantly increased in both species at maximum temperatures, with levels in C. gigas being 8-fold higher than in C. corteziensis. A significant loss of biomass and glycogen reserves stored in gonads was found in C. gigas at 34 °C. The mRNA signal of Hsp70 was detected in gonadic tissues from both oysters after thermal stress for in situ hybridization (ISH), with a temperature increase in both species; the cover area of Hsp70 was significantly higher in C. gigas during the experiment. Hemocyte infiltration significantly increased with increasing temperature in both oyster species, and apoptosis was strongly correlated with Hsp70 in both species (r = 0.93; P < 0.05). These results could explain the high tolerance that C. corteziensis has to thermal stress compared to C. gigas and could be used to adapt aquaculture strategies to the use of native species in subtropical climates to reduce summer mortality events.