Abstract Improving the stability of lead halide perovskite solar cells (PSCs) for industrialization is currently a major challenge. It is shown that moisture induces changes in global PSC performance, altering the nature of the absorber through phase transition or segregation. Understanding how the material evolves in a wet environment is crucial for optimizing device performance and stability. Here, the chemical and structural evolution of state‐of‐the‐art hybrid perovskite thin‐film Cs 0.05 (MA 0.15 FA 0.85 ) 0.95 Pb(I 0.84 Br 0.16 ) 3 (CsMAFA) is investigated after aging under controlled humidity with analytical characterization techniques. The analysis is performed at different scales through Photoluminescence, X‐ray Diffraction Spectroscopy, Cathodoluminescence, Selected Area Electron Diffraction, and Energy Dispersive X‐ray Spectroscopy. From the analysis of the degradation products from the perovskite layer and by the correlation of their optical and chemical properties at a microscopic level, different phases such as lead–iodide (PbI 2 ), inorganic mixed halide CsPb(I 0.9 Br 0.1 ) 3 and lead‐rich CsPb 2 (I 0.74 Br 0.26 ) 5 perovskite are evidenced. These phases demonstrate a high degree of crystallinity that induces unique geometrical shapes and drastically affects the optoelectronic properties of the thin film. By identifying the precise nature of these specific species, the multi‐scale approach provides insights into the degradation mechanisms of hybrid perovskite materials, which can be used to improve PSC stability.