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Marine heatwaves (MHWs) are prevalent in the South China Sea (SCS) and are primarily caused by the anomalous western Pacific subtropical high (WPSH) which suppresses the SCS summer monsoon. Our study investigates future summer MHWs in the SCS in response to the enhanced WPSH. The results show that the WPSH will be strengthened in most models under the highest emission scenario. Total days of summer SCS MHWs will significantly increase with the enhancement of the WPSH and peak at the end of the 21st century. The high‐value area of total days of summer MHWs (>50 days) appears in the south‐central SCS. The intensity of MHWs exhibits little response to the strengthening WPSH. In addition, the weakening of the SCS cold filament due to the anomalous WPSH also favors summer MHWs. Furthermore, increased future severe summer SCS MHWs will create a higher bleaching risk for coral reefs. Plain Language Summary: Marine heatwaves (MHWs) can have devastating impacts on marine ecosystems when seawater temperatures are warmer than the 90th percentile for at least five consecutive days based on a fixed 30‐year (or longer) baseline. The western Pacific subtropical high (WPSH) is an anticyclonic system in the northwestern Pacific that affects MHWs in the South China Sea (SCS). Based on the latest climate models, we calculate the total days and intensity of summer MHWs in the SCS under increased variability in the WPSH in future projections. Total days of summer MHWs will rapidly increase and peak at the end of the 21st century. Furthermore, the total days of summer MHWs are associated with SCS upwelling and cold filament, which is also influenced by strong WPSH. More concerning are the increases in severe summer MHWs, which will make coral reefs more susceptible to bleaching in the SCS in the future. Key Points: Increasing variability in the western Pacific subtropical high (WPSH) leads to more frequent, stronger WPSH events in future projectionsThe total days and intensity of summer marine heatwaves (MHWs) in the South China Sea (SCS) show different responses to the enhanced WPSHTotal days of summer MHWs in the SCS increase with the strengthened WPSH, resulting in a high coral bleaching risk [ABSTRACT FROM AUTHOR]

• Total days and average duration of marine heatwaves increased in coral reef zones. • The area ration between marine heatwaves and cold-spells is rapidly increasing. • Upper-ocean warming drives the differences between marine heatwaves and cold-spells. Persistent ocean temperature extremes, known as marine heatwaves (MHWs) and marine cold-spells (MCSs), can be devastating for warm-water coral reefs. However, the difference between MHWs and MCSs in the mean and trend across global coral reef zones in the present and future climates remains unclear. In this study, we comparatively assessed these changes using satellite sea surface temperatures during 1982–2021 and the Coupled Model Intercomparison Project Phase 6 climate model projections during 2022–2070. We found distinct differences in the mean and trend between MHW and MCS metrics across global coral reef zones, with a rapid increase in the total days and average duration of MHWs, whereas the MCSs exhibited an opposite trend. Moreover, the increasing trend in the average duration of MHWs and decreasing trend in MCS frequency both induced a nearly permanent MHW state in future projections. Upper-ocean warming was the main driver of the increase in MHWs and the decrease in MCSs. Under greenhouse warming, the area ratio between the MHW and MCS would rapidly increase from 1982 to 2070, reaching nearly 28 times under the Shared Socio-economic Pathway (SSP) 245 and 109 times under SSP585 by the year 2070. As El Niño-Southern Oscillation (ENSO) events are critical for the occurrence of MHWs and coral bleaching in tropical oceans, our results emphasize the necessity for understanding the projected changes in extreme ENSO events. [ABSTRACT FROM AUTHOR]