Although tropical cyclones (TCs) commonly induce rapid sea surface temperature (SST) cooling, delayed second SST cooling following TC landfall remains largely unexplored. Using OSTIA SST and ERA5 reanalysis data, we investigate second SST cooling associated with autumn landfalling TCs over the northwestern South China Sea shelf during 2006–2025. Eight of the 24 landfalling TCs (33.3%) exhibit a distinct two-stage SST evolution, characterized by an initial cooling, a brief recovery, and a subsequent second cooling. The second cooling typically develops 4–6 days after landfall, persists for 5–8 days, and reaches amplitudes of −0.5 to −3.8°C. In most cases, its magnitude exceeds that of the initial TC-induced cooling, although its cooling rate is generally slower. Compared with TCs without second cooling, second-cooling TCs tend to be stronger, more compact, and move faster. Composite analyses reveal that post-landfall large-scale atmospheric circulation adjustment promotes cold-air intrusion, which primarily drives the second cooling. Horizontal cold-water advection further enhances the SST decrease. These findings provide new insights into the delayed SST responses to TC landfall and improve our understanding of post-landfall air–sea interaction over marginal shelf seas.