Marine and atmospheric heatwaves are increasing in frequency, duration and intensity, exposing intertidal organisms to compound thermal extremes during immersion and emersion. Concerningly, marine and atmospheric heatwaves are likely to act as synergistic stressors for intertidal species such as the Sydney rock oyster, Saccostrea glomerata. We investigated the responses (survival, growth) of this species to a simulated dual marine (MHWs) and atmospheric (AHWs) heatwaves in context with heatwave conditions in the southeast Australia climate-change hotspot. Juveniles were exposed to an AHW (control air temperature 21°C, AHW, 27°C, 33°C) daily for six hours and then immersed in a MHW at the same control and treatment temperatures to simulate tidal cycles. This was done in two experiments of 15- and 21-days duration. Survival to the combined exposure to AHW and MHW depended on temperature. with high survival at the control temperature and at the moderate 27 °C and extreme 33°C heatwave. Exposure to the combined AHW at 33°C resulted in high mortality at two weeks. High survival of spat at 33°C for two weeks prior indicates thermal tolerance up to a point until the cumulative impacts of heat and aerial exposure pushed the spat of S. glomerata beyond their limits, with limited growth recorded. In their intertidal and shallow subtidal habitats MWHs and AHWS are a key risk to oysters as key foundation species with implications for reef restoration efforts.