Capillary barriers exhibit superior performance in arid regions; however, their performance in humid climates is limited by an insufficient water storage capacity. To address this limitation, this study explores the efficacy of a superabsorbent polymer (SAP) in enhancing the hydraulic performance of capillary barriers. The proposed system comprises an SAP-mixed fine-grained layer overlying a coarse-grained layer. One-dimensional soil column tests were conducted to characterize the unsaturated hydraulic properties of the SAP-mixed soil. The addition of 1% SAP increased the saturated volumetric water content by 55%, accompanied by a 23% increase in soil swelling. Rainfall simulations demonstrated that the SAP-enhanced upper layer effectively retained infiltrating water, maintaining a low pore water pressure. Notably, the barrier mixed with 0.75% SAP absorbed all precipitation without any percolation, which was attributed to the synergistic effects of increased water storage capacity and the capillary effect at the interface. However, percolation occurred in the case of 0.5% SAP when the storage capacity was exceeded. These findings indicate that 0.75% SAP is optimal for enhancing capillary barriers, enabling them to withstand a 24 h extreme rainfall event with a 100-year return period in Nanjing, China.