The South China Sea (SCS), as the largest marginal sea in the northwestern Pacific, is a key component of the Indo-Pacific ocean circulation system. Its upper-layer western boundary currents, including the winter Vietnam Coastal Current and the summer Vietnam Offshore Current, play important roles in regulating regional ocean dynamics, climate, and marine ecosystems. However, their long-term responses to global warming remain unclear because of limited observations and insufficient model resolution. In this study, we investigate changes in the upper-layer circulation of the SCS from 1951 to 2050 using an ensemble of 16 high-resolution climate model members. The results show that the SCS western boundary currents intensify under global warming in both winter and summer. The winter Vietnam Coastal Current increases by 7.7%, corresponding to an enhancement of 0.60 ± 0.47 Sv, while the summer Vietnam Offshore Current increases by 8.5%, corresponding to an enhancement of 1.10 ± 0.55 Sv. Further analysis based on a 1.5-layer reduced-gravity model indicates that enhanced upper-ocean stratification is the dominant driver of the winter current intensification. In summer, the strengthening of the Vietnam Offshore Current is mainly controlled by enhanced stratification and is partly influenced by intensified local wind stress curl. These results suggest that global warming can substantially alter tropical marginal-sea circulation through changes in ocean stratification and regional wind forcing. The intensified SCS western boundary currents may further influence heat transport, upwelling, nutrient redistribution, and regional climate variability in the Indo-Pacific domain.