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Under the combined impacts of natural changes and human activities, the past, current, and future marine heatwaves (MHWs) in China's marginal seas and adjacent offshore waters (CMSOW) need a comprehensive understanding. This study provides a systematic analysis of the spatiotemporal variations using daily sea surface temperature data and simulates the future trend using 12 climate models. During 1982–2018, the mean annual total days, duration, frequency, and mean intensity of the MHWs in the CMSOW increased by 20–30 days/decade, 5–9 days/decade, 1–2 decade−1, and 0.1–0.3°C/decade, respectively (p <0.01). The maximum sea surface temperature anomalies in the Bohai Sea was over 6–8°C, and the MHW's frequency, duration, and mean intensity were higher than twice the global average, which could have impacted fishery resources and occurrence of harmful algal blooms. The variations of the MHWs in the CMSOW result from the robust ocean surface warming, which is caused by increased solar radiation due to reduced cloud cover, reduced ocean heat loss from weaker wind speed, weakening but warmer Kuroshio, and strong El Niño. In the future, the areas with longer total days and duration will increase; the spatial pattern of frequency has a negative relationship with that of duration while that of mean intensity is mostly unchanged. Year 2040 is a key node for the future changes of MHW under different Representative Concentration Pathways. The trend of total days increases from fast to slow, and frequency shows an opposite trend; the duration and mean intensity rise faster after 2040.

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