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Large-scale concurrences of humid heatwaves substantially escalate the risks of heat-related mortality. However, quantifying the origins of these concurrent extremes remains challenging. Here we use a complex network approach applied to climate reanalysis data to show that the observed intensification of humid heatwaves is closely associated with coastal oceanic warming over the period 1982–2023. This linkage is more pronounced for the large-scale aggregation of extreme humid heatwaves than for the locally confined events. In particular, approximately 50% and 64% of the upward trends in humid heatwave frequency and spatial-aggregation strength over hotspot regions are linked to their adjacent oceans, respectively. These land–ocean linkages largely arise from tropical ocean-driven moisture transport towards land regions and from coupled terrestrial–oceanic warming in the mid-to-high latitudes associated with atmospheric Rossby waves. Compared with mid-to-high latitudes, the tropics encompass most high-risk areas and exhibit stronger land–ocean linkages, highlighting the critical role of tropical oceans. Climate model experiments further demonstrate the influence of tropical oceans on adjacent terrestrial humid heatwaves. Our study provides insights that coastal sea surface temperature can be a crucial precursor of the large-scale aggregation of humid heatwaves.