Pyridinium-3-carboxylic acid perchlorate was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurements show that this compound undergoes a reversible phase transition at approximately 135 K with a wide hysteresis of 15 K. Dielectric measurements confirm the transition at approximately 127 K. Measurement of the unit-cell parameters versus temperature shows that the values of the c axis and beta angle change abruptly and remarkably at 129 (2) K, indicating that the system undergoes a first-order transition at T(c) = 129 K. The crystal structures determined at 103 and 298 K are all monoclinic in P2(1)/c, showing that the phase transition is isosymmetric. The crystal contains one-dimensional hydrogen-bonded chains of the pyridinium-3-carboxylic acid cations, which are further linked to perchlorate anions by hydrogen bonds to form well separated infinite planar layers. The most distinct differences between the structures of the higher-temperature phase and the lower-temperature phase are the change of the distance between the adjacent pyridinium ring planes within the hydrogen-bonded chains and the relative displacement between the hydrogen-bonded layers. Structural analysis shows that the driving force of the transition is the reorientation of the pyridinium-3-carboxylic acid cations. The degree of order of the perchlorate anions may be a secondary order parameter.