Two-dimensional Ruddlesden−Popper (2DRP) perovskites have attracted intense research interest for optoelectronic applications, due to their tunable optoelectronic properties and better environmental stability than their three-dimensional counterparts. Furthermore, high-performance photodetectors based on single-crystal and polycrystalline thin-films 2DRP perovskites have shown great potential for practical application. However, the complex growth process of single-crystal membranes and uncontrollable phase distribution of polycrystalline films hinder the further development of 2DRP perovskites photodetectors. Herein, we report a series of high-performance photodetectors based on single-crystal-like phase-pure 2DRP perovskite films by designing a novel spacer source. Experimental and theoretical evidence demonstrates that phase-pure films substantially suppress defect states and ion migration. These highly sensitive photodetectors show I_light/I_dark ratio exceeding 3 × 10⁴, responsivities exceeding 16 A/W, and detectivities exceeding 3 × 10¹³ Jones, which are higher at least by 1 order than those of traditional mixed-phase thin-films 2DRP devices (close to the reported single-crystal devices). More importantly, this strategy can significantly enhance the operational stability of optoelectronic devices and pave the way to large-area flexible productions.