The recent rise of low‐dimensional Ruddlesden–Popper (RP) perovskites is notable for superior humidity stability, however they suffer from low power conversion efficiency (PCE). Suitable organic spacer cations with special properties display a critical effect on the performance and stability of perovskite solar cells (PSCs). Herein, a new strategy of designing self‐additive low‐dimensional RP perovskites is first proposed by employing a glycine salt (Gly+) with outstanding additive effect to improve the photovoltaic performance. Due to the strong interaction between CO and Pb2+, the Gly+ can become a nucleation center and be beneficial to uniform and fast growth of the Gly‐based RP perovskites with larger grain sizes, leading to reduced grain boundary and increased carrier transport. As a result, the Gly‐based self‐additive low‐dimensional RP perovskites exhibit remarkable photoelectric properties, yielding the highest PCE of 18.06% for Gly (n = 8) devices and 15.61% for Gly (n = 4) devices with negligible hysteresis. Furthermore, the Gly‐based devices without encapsulation show excellent long‐term stability against humidity, heat, and UV light in comparison to BA‐based low‐dimensional PSCs. This approach provides a feasible design strategy of new‐type low‐dimensional RP perovskites to obtain highly efficient and stable devices for next‐generation photovoltaic applications.