Marine heatwaves (MHWs), periods of anomalously warm sea surface temperature (SST) which can have significant impacts on marine ecosystems, have increased in frequency and severity over recent decades. Many coastal systems (e.g. coral reefs) are particularly vulnerable to warm-season heat stress when temperature can exceed organisms’ thermal thresholds and lead to mass mortality. While many studies have examined the change of the warm-season heat stress occurrence over time, e.g., for coral reefs, there has been less analysis of the thermal structure of heat stress events. Here we examined the trend in the characteristics of warm-season heat stress (referred to as warm-season MHWs) at the global-scale from 1985 to 2019, using multiple metrics for each of duration, peak intensity, accumulated heat stress, heating rates and level of intensity. The results showed that warm-season MHWs have become more frequent, longer-lasting, featured higher peak intensity and accumulated heat stress across most of the ocean over the past four decades. Furthermore, decomposition of the trends in warm-season MHWs structure showed that the increased accumulated heat stress was predominantly driven by the increased duration rather than the increased intensity, especially in the western and central equatorial Pacific. The results contribute to improving the understanding of warm-season MHWs, which may help inform the prediction of their impacts on marine ecosystems as well as marine conservation and management under climate change.