AbstractTailoring the organic spacing cations enables developing new Ruddlesden–Popper (RP) perovskites with tunable optoelectronic properties and superior stabilities. However, the formation of highly crystallized RP perovskites can be hindered when the structure of organic cations become complex. Strategies to regulate crystal growing process and grains quality remain to be explored. In this study, mixing Rb+ ions in precursor solution is reported to significantly promote the crystallinity of phenylethylammonium (PEA+) based RP perovskites without impacting on the major orientation of perovskite grains, which leads to increased power conversion efficiencies from 12.5% to 14.6%. It is found that the added Rb+ ions prefer to accumulate at crystal growing front and form Rb+ ions‐rich region, which functions as mild crystal growth inhibitor to retard the absorption and diffusion of organic cations at growing front and hence regulates crystal growing rate. The retarded crystal growth benefits PEA‐based RP perovskite films with elevated crystal qualities and prolonged carrier recombination lifetimes. Similar increased crystallinity and photovoltaic performance are achieved in other RP perovskites with non‐linear organic cations such as phenylmethylammonium (PMA+), 1‐(2‐naphthyl)‐methanammoniun (NMA+) by adding Rb+ ions, demonstrating using a small amount of growth inhibitor as a general route to regulate crystal growth.