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The onset of the summer monsoon associated with global warming is of great concern to the scientific community. While observational data diagnosis has shown the impact of intraseasonal oscillation (ISO) on the monsoon onset, how the ISO may affect the onset of monsoon under global warming remains unknown. Here, by analyzing the onset of the summer monsoon over the South Asian marginal seas projected by models in phase 6 of the Coupled Model Intercomparison Project (CMIP6) under the SSP5–8.5 scenario, we show evidence that the majority of models (>70%) project an earlier onset over the Arabian Sea (ArS) but a delayed onset over the Bay of Bengal (BoB) and the South China Sea (SCS). The temporal shifts of the monsoon onset are attributed to the changes in the premonsoon northward migration of equatorial ISO (NMISO), which is a trigger of monsoon onset and will be advanced (postponed) over the ArS (BoB and SCS). The subtropical upper-level westerly anomaly, inducing delayed occurrence of easterly shear, acts to delay the NMISO over the entire Indian Ocean. However, the intensified low-level southerly wind over the ArS, as well as its induced asymmetric pattern of boundary layer moisture work together to advance the premonsoon NMISO in the area, outweighing the delayed impact from vertical shear. These large-scale circulation changes are driven by tropical warming in the upper troposphere, land warming over the Arabian Peninsula, and ocean warming over the eastern Pacific. This analysis enriches monsoon onset projections by highlighting the role of ISO in influencing the future changes in monsoon onset. [ABSTRACT FROM AUTHOR]