Flexible perovskite solar cells (pero-SCs) are promising candidates to complement silicon photovoltaics; however, their stability remains far below industrial standards, particularly under long-term moisture exposure caused by water permeation through permeable plastic substrates. Although conventional hydrophobic interlayers can block water, they are generally incompatible with polar perovskite precursor solutions and are thus unsuitable for use beneath perovskite films. Here, a self-healing hydrophobic coating (SHC) is introduced that integrates a supramolecular polymer with dynamic imine cross-linking and SiO2 nanoparticles as a buried interfacial barrier. The SHC combines strong hydrophobicity, a low water vapor transmission rate, and exceptional self-healing ability, preserving its protective function even after surface treatment and perovskite deposition. Beyond suppressing moisture permeation, the SHC interacts with PbI2 to modulate the orientation of PbI2 layers, thus directing perovskite crystallization toward compact stacking, reduced residual PbI2, and preferential crystal orientation. Consequently, SHC-modified flexible pero-SCs achieve a record efficiency of 26.38% (25.74% certified) and 24.80% for small-area (0.062 cm2) and large-area (1.004 cm2) devices. The devices also demonstrate outstanding stability, maintaining 81.18% of their initial efficiency after 1000 h at 85% relative humidity and passing the IPX7 waterproofing standard under complete immersion.