The significant open-circuit voltage (VOC) deficit poses a major obstacle to enhancing the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Interface passivation emerges as a potent strategy to regulate carrier transport and boost performance. Here, we innovatively introduced rare-earth lanthanum (La) to passivate the absorber interface by directly treating the absorption layer surface with an aqueous La3+ ion solution. This approach effectively minimizes interface defect concentrations and mitigates Fermi-level pinning effects. Notably, the VOC markedly increases from 406 to 456 mV after La treatment. Consequently, the power conversion efficiency soars from 6.78% (VOC = 406 mV, JSC = 29.95 mA/cm2, FF = 55.28%) for the reference cell to 7.89% (VOC = 451 mV, JSC = 30.12 mA/cm2, FF = 59.56%) for the optimized La-processed cell. This groundbreaking work opens up a novel avenue for advancing CZTSSe solar cell performance, offering promising implications for the future of CZTSSe thin-film photovoltaic technology.