Abstract To explore multidecadal trends of the Vietnamese Southern Coastal Upwelling (SCU), we employed an SCU index (Southern Coastal Upwelling index [SCUI]), defined as the difference between basin‐wide SST (Tref ${T}{mathrm{r}mathrm{e}mathrm{f} }$) and local minimum SST (Tmin ${T}{min }$). During 1982–2024, SCUI shifted markedly around 1999, weakening before 1999 (B99) and intensifying thereafter (A99). Empirical Orthogonal Function (EOF) analyses identify two key dynamical processes: (a) a persistent La Niña–like SST pattern accompanied by intensified Pacific trade winds, enhancing northern South China Sea (SCS) anomalous anticyclonic circulation and reducing SCS Throughflow; and (b) multidecadal variability in interhemispheric thermal contrast, resulting in cross‐equatorial winds. Under their joint influence, the B99 weakening was dominated by a reduced southwesterly summer monsoon affecting Tmin ${T}{mathit{min} }$ regulated by trade winds and cross‐equatorial winds, while the A99 intensification was attributable to Tref ${T}{ref}$ warming induced by enhanced northern SCS negative wind stress (WS) curl anomaly and weakened SCS Throughflow rather than enhanced local upwelling itself.