An intriguing reversible band gap narrowing behavior of the lead‐free hybrid perovskite single crystal DMASnI3 (DMA=CH3NH2CH3+) from yellow to black is observed without phase transformation. We discuss the transformation mechanism in detail. More interestingly, the transformed samples in black can rapidly self‐heal into yellow ones when exposed to deionized water (DI water). Contrary to other hybrid perovskites, DMASnI3 crystals exhibit excellent water phase stability. For example, DMASnI3 was immersed in DI water for 16 h and no decomposition was observed. Inspired by its excellent water phase stability, we demonstrate a potential eco‐friendly application of DMASnI3 in photo‐catalysis for H2 evolution in DI water. We present the first H2 evolution rate of 0.64 μmol h−1 with good recycling properties for pure DMASnI3 crystals. After the narrowing process, the optical band gap of DMASnI3 can be lowered from 2.48 eV to 1.32 eV. Systematical characterizations are applied to investigate their structures and optoelectronic properties. The reversible band gap narrowing behavior and outstanding electrical properties, such as higher carrier mobility and long carrier lifetime show that DMASnI3 has a great potential for optoelectronic applications. Reversible band gap narrowing behavior of Sn‐based hybrid perovskite single crystal was first revealed with excellent phase stability, narrow band gap, higher carrier mobility and longer carrier lifetime. This is encouraging for the optoelectronic application.