Precisely engineered semiconductor heterojunctions with tunable morphologies are emerging as efficient systems for solar energy conversion. We report a rationally designed 3D hollow ZnCo2S4 polyhedron, derived from MOFs and intimately coupled with 2D g-C3N4 (CN) nanosheets via a hydrothermal route. The formation of an S-scheme ZnCo2S4–CN heterojunction significantly enhances light absorption and accelerates charge carrier separation. Structural and spectroscopic characterizations confirm the strong interfacial coupling, which induces an internal built-in electric field and Coulombic interactions that promote directional charge transfer while maintaining the strong redox potentials of each component. Owing to these synergistic effects, the optimized 20 wt % ZnCo2S4–CN composite achieves an exceptional hydrogen evolution rate of 2390 μmol g–1 h–1 under visible-light irradiation, which is 57 and 13 times higher than that of pristine CN and ZnCo2S4, respectively. Moreover, the composite exhibits excellent stability and recyclability over prolonged photocatalytic cycles. This work highlights that engineering sulfur-vacancy-rich, MOF-derived sulfides on CN is an effective strategy for constructing high-performance heterojunction photocatalysts for sustainable hydrogen production.