Although power conversion efficiency (PCE) of state‐of‐the‐art perovskite solar cells has already exceeded 20%, photo‐ and/or moisture instability of organolead halide perovskite have prevented further commercialization. In particular, the underlying weak interaction of organic cations with surrounding iodides due to eight equivalent orientations of the organic cation along the body diagonals in unit cell and chemically non‐inertness of organic cation result in photo‐ and moisture instability of organometal halide perovskite. Here, a perovskite light absorber incorporating organic–inorganic hybrid cation in the A‐site of 3D APbI3 structure with enhanced photo‐ and moisture stability is reported. A partial substitution of Cs+ for HC(NH2)2+ in HC(NH2)2PbI3 perovskite is found to substantially improve photo‐ and moisture stability along with photovoltaic performance. When 10% of HC(NH2)2+ is replaced by Cs+, photo‐ and moisture stability of perovskite film are significantly improved, which is attributed to the enhanced interaction between HC(NH2)2+ and iodide due to contraction of cubo‐octahedral volume. Moreover, trap density is reduced by one order of magnitude upon incorporation of Cs+, which is responsible for the increased open‐circuit voltage and fill factor, eventually leading to enhancement of average PCE from 14.9% to 16.5%. FA0.9Cs0.1PbI3 with improved moisture‐ and photostability is developed. Incorporation of 10% of Cs cation in the FA cation sites improves photovoltaic performance as well as photo‐ and moisture stability. Property–structure correlation plays important role in improving the stability of perovskite solar cells.