Climate-driven atmospheric heatwaves (AHWs) are increasing in frequency and intensity across Australia and exposing intertidal species such as the Sydney rock oyster (Saccostrea glomerata) to extreme aerial temperatures. These events have been linked to mortality and disease outbreaks, yet their effects on oyster immune function remain underexplored. This study synthesises findings from two controlled experiments examining the effects of simulated AHWs (42 °C, 6 h day⁻¹ over three days) on oyster immune responses. Haemocyte counts, phagocytosis, and condition index were measured after each exposure as an assay of immunophysiological response. AHW exposure consistently compromised immune performance, characterised by decreased total haemocyte counts and cessation of phagocytic activity while intertidal acclimatisation (low vs high shore emersion) did not buffer oysters against AHW-induced immune decline. Notably, AHW caused immediate and sustained immunosuppression with no cumulative decline over time, suggesting a rapid onset of impairment. Despite stable condition indices and low mortality, suppressed immune function may increase susceptibility to disease under future climate scenarios. These results highlight the vulnerability of Saccostrea glomerata populations to short-term extreme heat events and understanding these findings provide valuable insight into how climate stressors may impact the resilience of the ecologically, economically and culturally significant Sydney rock oysters.