AbstractSulfide kesterite Cu₂ZnSnS₄ (CZTS) solar cells, containing earth‐abundant and environmentally benign constituents, are regarded as promising candidates for thin‐film photovoltaic technologies. CZTS device performance, however, is currently limited by severe nonradiative recombination caused by abundant deep‐level defects. Herein, an effective defect engineering approach for high bandgap CZTS solar cells using a newly introduced moisture‐assisted post‐deposition annealing treatment is reported. This treatment modifies the local chemical composition within the heterojunction and CZTS grain boundaries and enhances the incorporation of Cd within the CZTS layer during CdS deposition. Cd not only accumulates at the grain boundaries, but it also presents in grain interiors where it occupies Cu lattice sites. The overall modification of the local chemical environment suppresses deep level defects and activates relatively shallow acceptor Cu_Zn antisites and Cu vacancies, giving rise to remarkably improved device performance. This work opens a new direction for defect engineering of kesterite materials, which may also be applicable to other thin film semiconductors.