AbstractSolid-state refrigeration based on the caloric effect is viewed as a promising efficient and clean refrigeration technology. Barocaloric materials were developed rapidly but have since encountered a general obstacle: the prominent caloric effect cannot be utilized reversibly under moderate pressure. Here, we report a mechanism of an emergent large, reversible barocaloric effect (BCE) under low pressure in the hybrid organic–inorganic layered perovskite (CH₃–(CH₂)_n−1–NH₃)₂MnCl₄ (n = 9,10), which show the reversible barocaloric entropy change as high as ΔS_r ∼ 218, 230 J kg⁻¹ K⁻¹ at 0.08 GPa around the transition temperature (T_s ∼ 294, 311.5 K). To reveal the mechanism, single-crystal (CH₃–(CH₂)_n−1–NH₃)₂MnCl₄ (n = 10) was successfully synthesized, and high-resolution single-crystal X-ray diffraction (SC-XRD) was carried out. Then, the underlying mechanism was determined by combining infrared (IR) spectroscopy and density function theory (DFT) calculations. The colossal reversible BCE and the very small hysteresis of 2.6 K (0.1 K/min) and 4.0 K (1 K/min) are closely related to the specific hybrid organic–inorganic structure and single-crystal nature. The drastic transformation of organic chains confined to the metallic frame from ordered rigidity to disordered flexibility is responsible for the large phase-transition entropy comparable to the melting entropy of organic chains. This study provides new insights into the design of novel barocaloric materials by utilizing the advantages of specific organic–inorganic hybrid characteristics.