A layered organic-inorganic hybrid compound, tetra(cyclopentylainmonium) decachlorotricadmate(II) (1), in which the two-dimensional [Cd3Cl10](n)(4-) networks built up from three face-sharing CdCl6 octahedra are separated by cyclopentylammonium cation bilayers, has been discovered as a new phase transition material. It undergoes two successive structural phase transitions, at 197.3 and 321.6 K, which were confirmed by differential scanning calorimetry measurements, variable-temperature structural analyses, and dielectric measurements. The crystal structures of 1 determined at 93, 298, and 343 K are solved in P2(1)2(1)2(1), Pbca, and Cmca, respectively. A precise analysis of the structural differences between these three structures reveals that the origin of the phase transition at 197.3 K is ascribed to the order-disorder transition of the cyclopentylammonium cations, while the phase transition at 321.6 K originates from the distortion of the two-dimensional [Cd3Cl10](n)(4-) network.