Tin-halide perovskites (THPs) have emerged as promising lead-free perovskites for photovoltaics and photocatalysis applications but still fall short in terms of stability and efficiency with respect to their lead-based counterpart. A detailed understanding of the degradation mechanism of THPs in a water environment is missing. This Letter presents ab initio molecular dynamics (AIMD) simulations to unravel atomistic details of THP/water interfaces comparing methylammonium tin iodide, MASnI3, with the lead-based MAPbI3. Our results reveal facile solvation of surface tin–iodine bonds in MASnI3, while MAPbI3 remains more robust to degradation despite a larger amount of adsorbed water molecules. Additional AIMD simulations on dimethylammonium tin bromide, DMASnBr3, investigate the origins of their unprecedented water stability. Our results indicate the presence of amorphous surface layers of hydrated zero …