The structure and the dielectric properties of Pb (Zn_1/3Nb_2/3)O₃ (PZN) crystal have been investigated by means of high-resolution synchrotron x-ray diffraction (with an x-ray energy of 32 keV) and dielectric spectroscopy (in the frequency range 100 Hz–1 MHz). At high temperatures, the PZN crystal exhibits a cubic symmetry and polar nanoregions inherent to relaxor ferroelectrics are present, as evidenced by the single (222) Bragg peak and by the noticeable tails at the base of the peak. At low temperatures, in addition to the well-known rhombohedral phase, another low-symmetry, probably monoclinic, phase is found. The two phases coexist in the form of mesoscopic domains. The ferroelectric phase transition is diffuse and observed between 325 and 390 K, where the concentration of the low-temperature phases gradually increases and the cubic phase disappears upon cooling. However, no dielectric anomalies can be detected in the temperature range of the diffuse phase transition. The temperature dependence of the dielectric constant shows a maximum at higher temperature (T_m = 417–429 K, depending on frequency) with the typical relaxor dispersion at T<T_m and the frequency dependence of T_m fitted to the Vogel–Fulcher relation. Application of an electric field upon cooling from the cubic phase or poling the crystal in the ferroelectric phase gives rise to a sharp anomaly of the dielectric constant at  K and greatly diminishes the dispersion at lower temperatures, but the dielectric relaxation process around T_m remains qualitatively unchanged. The results are discussed in the framework of the present models of relaxors and in comparison with the prototypical relaxor ferroelectric Pb (Mg_1/3Nb_2/3)O₃.