Realizing high‐quality perovskite films through uniform defect passivation and crystallization control is pivotal to unlocking the potential of scalable applications. However, prevalent small‐molecule additives are inherently susceptible to the crystallization dynamics of perovskites, resulting in non‐uniform distribution within the crystalline film and impeding consistent passivation and precise crystallization control. While polymers offer improved uniformity, their poor solubility restricts practical applications. To overcome this limitation, an in situ self‐polymerization strategy is employed, enabling homogeneous coordination between sulfonate‐containing additives and undercoordinated lead cations. This approach enhances perovskite film quality, promotes larger crystalline grain domains, and facilitates more efficient charge transport across grain domain boundaries. As a result, perovskite solar cells (PSCs) achieve a remarkable power conversion efficiency of 25.34% in small‐area devices and 21.54% in 14.0 cm 2 mini‐modules, accompanied by exceptional operational stability. These findings highlight in situ polymerization as an effective strategy for leveraging sulfonate additives to overcome distribution challenges, advancing the scalable fabrication of efficient and stable PSCs.