A novel lead-free 2D perovskite, namely PEA₂SnBr₄, shows impressive water-resistance by retaining its original crystal structure and optical properties when placed in contact with water. Such key properties have been advantageously used for the fabrication of a novel co-catalytic system by coupling PEA₂SnBr₄ with graphitic carbon nitride. PEA₂SnBr₄/g-C₃N₄ composites at different metal halide perovskite loadings (5 and 15 wt%) have been prepared and tested in hydrogen photogeneration in an aqueous environment and organic dye degradation (methylene blue). The results show an impressive enhancement of H₂ production of the composite with respect to the two separate components with hydrogen evolution rates up to 1600 μmol g⁻¹ h⁻¹ and analogous improvements in the efficiency of methylene blue degradation. The present results, providing a novel water-resistant perovskite and co-catalytic system, pave the way towards the safe, efficient and real use of metal halide perovskites in catalysis.