Calorimetry, high-pressure dilatometry, X-ray powder diffraction and electric conduction studies were applied to investigate the phase transitions and physical properties of guanidinium nitrate, (C(NH₂)3)⁺NO₃^-. Two thermal anomalies related to the first-order and second-order phase transitions were observed at T₁₂ = 296 K and T₂₃ = 384 K, respectively. The transformation at T₂₃ indicates the existence of a new high-temperature phase of guanidinium nitrate, which has been confirmed by the X-ray powder diffraction study. The transition entropies were found to be Delta S₁₂ = 5.7 J mol^-1 K and Delta S₂₃ = 1.5 J mol^-1 K. The magnitude of the change in the entropy at the 296 K phase transition indicates its order-disorder character. The change in the specific volume at T₁₂ is negative, i.e. Delta V₁₂ = -1.6 cm³ mol^-1, and T₁₂ linearly decreases with increasing pressure, with dT₁₂/dp = -24.3 K kbar^-1. The deuterization has only a slight effect on the temperatures of the observed phase transitions. Above 320 K an increase in electric conductivity has been noted in the polycrystalline samples of the compound studied. The results of the studies performed suggest that the mechanism of the phase transition at 296 K, accompanied by unusual deformation of the crystals, may be of an electrostatic nature.