Convection initiation (CI) and the subsequent upscale convective growth (UCG) at the coast of South China in a warm-sector heavy rainfall event are shown to be closely linked to a varying marine boundary-layer jet (MBLJ) over the northern South China Sea (NSCS). To elucidate the dynamic and thermodynamic role of the MBLJ in CI and UCG, we conducted and analyzed convection-permitting numerical simulations and observations. Compared to radar observations, the simulations capture CI locations and the following southwest–northeast-oriented convection development. The nocturnal MBLJ peaks at 950 hPa and significantly intensifies with turning from southwesterly to nearly southerly by inertial oscillation. The strengthened MBLJ promotes mesoscale ascent on its northwestern edge and terminus where enhanced convergence zones occur. Located directly downstream of the MBLJ, the coastal CI and UCG are dynamically supported by mesoscale ascent. From a thermodynamic perspective, a warm-moist tongue over the NSCS is strengthened by the MBLJ-driven mesoscale ascent as well as by a high sea-surface temperature. The warm moist tongue further extends northeastward by horizontal transport and arrives at the coast where CI and UCG occur. Near the CI location, rapid development of a low-level saturated layer is mainly attributed to the mesoscale ascent and low-level moistening associated with the MBLJ. In addition, subsequent CI happens on either side of the original CI along the coast due to the delay of low-level moistening, which partly contributes to linear convective growth. Furthermore, ensemble simulations confirm that a stronger MBLJ is more favorable to CI and UCG near the coast.