Three-dimensional (3D) inorganic–organic hybrid perovskites have attracted considerable attention during the past decade because of their superior optoelectronic properties and broad application prospects, especially in energy-related fields. However, the applications of 3D perovskites, for example, the well-studied CH₃NH₃PbI₃, are severely restrained by their environmental instability, photoinstability, and crystal processing difficulties. In contrast to their 3D counterparts, Ruddlesden–Popper phases, which are layered two-dimensional (2D) perovskites, have shown promising stability and excellent performance in photodetection and solar cell applications, which can be achieved through appropriate selection of organic spacer cations and artificial tuning of the number of perovskite-like layers. Because of their decreased dimensions, novel properties also appear in Ruddlesden–Popper perovskites, such as large exciton binding energy, sensitive photodetection, high photoluminescence quantum yield, and a wide bandgap. In light of this, 2D Ruddlesden–Popper perovskites have garnered much interest in recent years and various methods have been developed for their synthesis, property evaluation, and device fabrication. In this review, recent progress in the synthesis and enhanced ambient stability of 2D Ruddlesden–Popper perovskites is summarized. The applications of these materials in advanced photodetectors are emphasized and challenges limiting their ongoing development are also discussed.