Marine heatwaves (MHWs) have adverse impacts on marine ecosystems and fisheries, yet extreme MHWs in the western tropical Pacific Ocean remain largely unexplored. Here we examine the evolution and drivers of the two strongest subsurface MHWs in the western tropical Pacific Ocean on record, in 2008 (MHW-08) and 2010–2011 (MHW-11). The two events have similar evolutions, as they were initiated and developed locally in tropical regions during their onset and both gradually dissipated with multiple cores during the decay phase. We examined the physical processes during the evolution of MHW-08 and MHW-11, and performed a heat budget analysis for the subsurface layer in each case. We find that the onset and decay of both events were mainly controlled by convergence/divergence due to anomalous Ekman downwelling/upwelling and accompanying anomalous horizontal currents. The development of the two events was dominated by vertical heat advection, while horizontal advection played an essential additional role during the decay. The relationship between the subsurface MHWs and El Niño-Southern Oscillation is discussed. Anomalous heat advection under the background warming caused by La Niña events lead to the extreme subsurface MHW events. Our findings may have important implications for environmental change and marine life in the western tropical Pacific Ocean.