Although great progress is being made toward improving the power conversion efficiency (PCE) and the operational stability of perovskite solar cells (PSCs), little attention is being paid to their mechanical reliability, which is particularly important for flexible PSCs (f-PCSs). Here, we have grown low-dimensional (LD) metal-halide perovskite (MHP) thin capping layer over the 3D MHP light-absorber thin film in f-PSCs in situ to improve their mechanical reliability. This resulted in f-PSCs with unprecedented, simultaneous improvements in PCE (21.0%), operational stability (T90, retention of 90% of the initial PCE, >800 h), and bending durability (T80 = 20,000 tensiononly bending cycles). These synergistic virtuous effects are attributed to the LD MHP capping layer enhancing photocarriers extraction, providing protection against the environment, and "sealing" surface flaws on the 3D MHP thin film. The latter reduces the pro. pensity for cracking in bending, which results in improved environmental stability and bending durability.