<p>Tin perovskites have emerged as promising lead-free alternatives for eco-friendly perovskite solar cells; however, their device performance still lags behind that of lead-based counterparts, mainly due to poor thin-film quality, high defect densities, and the intrinsic instability Sn²⁺. Here, we report a resonance-induced electron delocalization strategy to fundamentally stabilize tin perovskite by enhancing Sn-centered interactions using trithiocyanuric acid (Cy-SH) and cyanuric acid (Cy-OH) molecules as interfacial layer. We further demonstrate the electron delocalization between the N–C = O and N = C–OH tautomeric configurations of Cy-OH at the perovskite buried interface, which strengthens its interaction with uncoordinated Sn atoms, thereby significantly suppressing non-radiative recombination and enhancing carrier extraction and transport. As a result, the best-performing device achieves a power conversion efficiency (PCE) of 12.1%, maintaining 86% of its initial efficiency after 1000 h aging in N₂ containing 50-100 ppm O₂. This work demonstrates that tautomeric molecules can be rationally designed and utilized to regulate tin perovskite crystallization and defect states, thereby providing guidelines for the development of high-performance lead-free photovoltaics.</p>