Propagation patterns of 30- to 90-day boreal summer intraseasonal oscillation (BSISO) markedly affect subseasonal prediction. We show that global warming significantly alters BSISO propagation by Coupled Model Intercomparison Project Phase 6 projections. Under the Shared Socioeconomic Pathway 5-8.5 scenario, the eastward mode’s expansion propagation speed shows a twofold increase from 5.1 ± 1.9 to 10.3 ± 2.0 meters per second by the late 21st century, with a remarkable eastward extension by ~30° longitude compared to the present climate. The accelerated eastward mode frequency is expected to rise by 15%, intensifying subseasonal precipitation whiplash worldwide and shortening disaster preparedness time. This acceleration results from an asymmetric pattern of boundary layer moisture convergence anomalies, driven by weakened Rossby wave westerlies and enhanced Kelvin wave response linked to warming-induced atmospheric stabilization and El Niño–like ocean surface warming pattern. Conversely, a projected weakening of easterly vertical shear over the northern Indian Ocean attenuates the Rossby wave response and favors more eastward expansion events.