<p>The summer heatwave (HW) in Indochina is a growing hazard for local communities and is strongly associated with variations in the South Asian summer monsoon (SASM). Since the diabatic heating plays a crucial role in driving the SASM, this study investigates the interannual covariation between the summer heatwave frequency (HWF) in Indochina and the column-integrated diabatic heating (CIDH) over South Asia during the period 1981–2017 using the observation and reanalysis datasets. Singular value decomposition (SVD) analysis reveals a leading mode (SVD1) featuring consistent changes between CIDH over South Asia and the HWF in Indochina in summer at the interannual timescale. When the CIDH intensifies over the southern Indian subcontinent, an anomalous U-shaped vertical circulation is induced across South Asia and Indochina, resulting in anomalous descent in Indochina. The diabatic and adiabatic heatings associated with this descent in Indochina modulate local surface air temperature variations that further influence the HWF. The CIDH anomalies in summer are driven by the delayed effect of sea surface temperature (SST) over the northern Indian Ocean. In spring, negative CIDH anomalies over the northern Indian Ocean trigger an anticyclonic anomaly, reducing surface winds and surface evaporation. The reduction in latent-heat release favors an increase in the SST over the tropical Indian Ocean. Positive SST anomalies persist into summer and induce positive CIDH anomalies over South Asia. The above mechanism related to the development of CIDH anomalies from spring to summer and their impact on the summer HWF in Indochina are supported by numerical experiments.</p>