A cubic perovskite of complex composition Pb _{1 / 3} Na _{1 / 3} K _{1 / 3} Ta _{2 / 3} Fe _{1 / 3} O _3−δ (sp. gr. Pm-3m, a = 3.9767(3) Å) was synthesized for the first time by the solid-phase reaction method. The samples are characterized by an almost pore-free microstructure formed by slightly melted randomly oriented cubic crystallites. The band gap (≈2.05 eV) of a complex perovskite for a direct allowed electronic transition was calculated from the data of the diffuse reflection spectrum. According to impedance spectroscopy data, three polarization processes are observed in the sample at room temperature: low-frequency (at a frequency less than 300 Hz), medium-frequency (from 300 Hz to 10 kHz), and high-frequency (from 100 kHz to 10 MHz) ones. The permittivity of the sample at room temperature and a frequency of 10 ⁷ Hz reaches high values of about 147, and the dielectric loss tangent does not exceed 0.12.