AbstractTin‐halide perovskite solar cells (THPSCs), while offering low toxicity and high theoretical power conversion efficiency, suffer from inferior device performance compared to lead‐based counterparts. The primary limitations arise from challenges in fabricating high‐quality perovskite films and mitigating the oxidation of Sn2+ ions, which leads to severe non‐radiative voltage losses. To address these issues, we incorporate the rare‐earth element erbium chloride (ErCl3) into PEA0.15FA0.70EA0.15SnI2.70Br0.30 perovskite to effectively control the nucleation and crystal growth, significantly influencing the morphology of the perovskite films. As a result, the ErCl3‐processed THPSC exhibits an impressive open‐circuit voltage (VOC) of 0.83 V and power conversion efficiency (PCE) of 14.0% with the superior light and air stability, compared to the control device (VOC = 0.77 V and PCE = 12.8%). This ErCl3‐strategy provides a feasible solution for high‐performance THPSCs by regulating nucleation/crystallization kinetics and mitigating excessive crystal defects during the preparation process of lead‐free perovskites.image