Formamidinium lead iodide (FAPbI₃) perovskitesPerovskites” hold great promise in next-generation optoelectronic devices due to their superior optical and electronic propertiesProperties. However, their long-term environmental stability—particularly against moisture—remains a critical challenge. In this study, we employed ab initio molecular dynamics (AIMD) simulationsSimulation using the CP2K package to investigate the interaction between a FAPbI3 perovskitePerovskites slab and water molecules. We explored the structural evolution of the water/perovskitePerovskites interfaceInterface under thermal conditions to understand the initial stages of degradation. FAPbI3 with two different termination systems has been considered for this study to assess their effect. The simulationsSimulation reveal that water molecules, which initially reside above the surface, gradually adsorb toward the lattice and form coordination bonds with undercoordinated surface Pb2⁺ ions in the PbI2-terminated system. This Pb–O bonding indicates the onset of chemical degradation, leading to distortion of the PbI6 octahedral network. AIMD results show that water forms hydrogenHydrogen bonds with FA⁺ cations on the FAI-terminated FAPbI3 surface, offering partial protection but still allowing gradual moisture-driven degradation. Water exposure widens the bandgap of FAPbI3, flattens its bands, and alters k-point symmetry, thereby reducing charge transport efficiency compared to pure FAPbI3. Our findings provide a fundamental understanding of moisture-induced degradation mechanisms in FAPbI3 and highlight the importance of surface engineering to enhance perovskitePerovskites device stability under humid conditions.