AbstractPerovskite solar cells (PSCs) have shown rapid development recently, whereas nonideal stability remains the chief obstacle toward commercialization. Thus, it is of utmost importance to probe the degradation pathway for the entire device. Here, the extrinsic stability of inverted PSCs (IPSCs) is investigated by using standard shelf‐life testing based on the International Summit on Organic Photovoltaic Stability protocols (ISOS‐D‐1). During the long‐term assessment of 1700 h, the degraded power conversion efficiency is mainly caused by the fill factor (53% retention) and short‐circuit current density (71% retention), while the open‐circuit voltage still maintains 97% of the initial values. Further absorbance evolution and density functional theory calculations disclose that the perovskite rear‐contact side, in particular for the perovskite/fullerene interface, is the predominant degradation pathway. This study contributes to understanding the aging mechanism and enhancing the durability of IPSCs for future applications.