2D perovskites based on Formamidinium (FA) hold the potential for excellent stability and a broad absorption range, making them attractive materials for solar cells. However, FA-based 2D perovskites produced via one-step processing exhibit poor crystallinity and random quasi-quantum wells (QWs), leading to subpar photovoltaic performance. In this study, a seed-induced growth approach is introduced employing MAPbCl and BDAPbI in the deposition of FA-based Dion-Jacobson 2D perovskite films. This method yields high-quality perovskite films as the seeds preferentially precipitate and serve as templates for the epitaxial growth of FA-based counterparts, effectively suppressing the δ phase. Moreover, the epitaxial growth facilitated by uniformly dispersed seeds results in simultaneous crystallization from top to bottom, efficiently mitigating random phases (n = 2, 3, 4…) induced by the diffusion of organic cations and, in turn, minimizing energy loss. The impact of seed-induced growth on the crystallization and phase distribution of FA-based 2D perovskites is systematically investigated. As a result, the optimized FA-based 2D perovskite solar cell delivers an outstanding efficiency of 20.0%, accompanied by a remarkable fill factor of 0.823. Additionally, the unencapsulated device demonstrates exceptional stability, maintaining 98% of its initial efficiency after 1344 h of storage.