Flexible perovskite solar cells (PSC) represent a viable technology for applications requiring high power‐to‐weight ratios, seamless integration onto a variety of shapes, and low embedded energy. Strategies to boost the performance of flexible PSC, which still lag behind their rigid counterparts, include passivation of the perovskite surface with ammonium salts. Herein, the optimization of low‐temperature solution‐processed phenethylammonium iodide (PEAI) layers to passivate triple‐cation perovskite films as an effective strategy for flexible planar PSCs is reported. The best PEAI‐passivated cell delivers a maximum power conversion efficiency of 15.2% in forward scan, which is 22% higher in relative terms compared with nonpassivated devices, due to a major improvement in both open‐circuit voltage and fill factor. Optoelectrical characterization highlights that PEAI films effectively reduce trap‐mediated charge recombination, without hindering charge transport across the perovskite/hole transport layer interface. Light‐soaking stability of nonencapsulated cells with PEAI layers is also greatly enhanced, retaining 80% of the initial power conversion efficiency for twice as long as the reference devices, therefore proving the potential of this material for the development of efficient and durable flexible PSCs.