In recent years, the Yangtze River Valley (YRV) has become a key region of increasing summer heatwave risk in East Asia. During 1979–2023, both summer heatwave frequency and maximum temperature over the YRV increased significantly. Previous studies have often emphasized the role of the Western Pacific Subtropical High in East Asian summer heatwaves, but our analyses suggest that the South Asian High (SAH) provides a more robust upper-tropospheric pathway for the recent sustained growth of YRV heatwaves. The SAH shifted notably to the east from approximately 115°E in the climatological state to roughly 130°E in the trend-adjusted state. This eastward extension was associated with upper-tropospheric high-pressure anomalies and strengthened subsidence over the YRV. Meanwhile, the eastward extension of the SAH is closely associated with enhanced rainfall and ascending motion near the northwestern edge of the Indian summer monsoon region, suggesting strengthened convection and latent heating over South Asia. The associated upper-level flow extends eastward and connects with the descending motion over the YRV, indicating a possible east–west overturning circulation between South Asia and East Asia.The descending branch over the YRV favors adiabatic warming and enhanced surface shortwave radiation, thereby providing favorable conditions for persistent high temperatures.This pathway offers a potential mechanism for understanding the role of spatial changes of ISM in East Asian climate.