AbstractThis study aims to investigate the impact of submesoscale vertical advection (SVA) on the primary productivity in the southern East China Sea. The analysis is based on a comparison between two numerical simulations by using a three‐dimensional coupled physical‐biogeochemical model. One simulation directly resolves SVA on a high‐resolution mesh, and the other leaves SVA unresolved on a low‐resolution mesh. The high‐resolution simulation outperforms the low‐resolution simulation in reproducing the observed chlorophyll distribution, particularly in summer. Resolving SVA results in an approximately 40% increase in primary productivity during the summer, though SVA activity is relatively weak in this season than in other seasons. Among multiscale physical processes, SVA, rather than mixing, is found to be the most important vertical nutrient supply pathway from the nutrient‐rich bottom water to the nutrient‐depleted surface water in summer, particularly on the middle and outer shelves. The impact of SVA on the shelf is unique compared to the open ocean in that it efficiently enhances vertical supply of nutrient‐rich subsurface waters to the nutrient‐depleted surface layer. This study highlights the importance of SVA in promoting primary productivity in stratified shelf seas.