Marine heatwaves in the summertime when temperatures may exceed organisms’ thermal thresholds (“warm-season MHWs”) have huge impacts on the health and function of ecosystems like kelp forests and coral reefs. While previous studies showed that MHWs are likely to become more frequent and severe under future climate change, there has been less analysis of the thermal properties of warm-season MHWs or on the effects of climate model biases on these projections. In this study, we examine CMIP6 model ability to simulate five key thermal properties of warm-season MHWs, and evaluate the global pattern of future projections for coral reef and kelp systems. The results show that the duration, accumulated heat stress and peak intensity are projected to increase by > 60 day, 160 °C·day and 1 °C, respectively, across most of the ocean by the end of 21st century. In contrast, the duration of “priming” (a period of sub-lethal heat stress prior to MHW development) is projected to decrease by > 30 day in the tropics, potentially reducing organisms’ ability to acclimate to heat stress. The projected increases in the MHW duration and accumulated heat stress in some coral reef and kelp forest locations, however, are likely overestimated due to model limitations in simulating surface winds, deep convections and some other processes that influence MHW evolution. The findings point to the processes to target in model development and regional biases to be considered when projecting the impacts of MHWs on marine ecosystems.