Bismuth layer-structured Na_0.5Gd_0.5Bi₄Ti₄O₁₅ ceramics were synthesized via a high temperature solid state reaction. The samples were annealed under pure-argon and pure-oxygen atmospheres respectively. The dielectric relaxation and electrical properties were investigated in a wide range of frequencies and temperatures by dielectric/impedance spectroscopies. Two anomalies were observed in the temperature dependent dielectric spectra, one occurred in the range of ∼200–400 °C and the other was at ∼590 °C due to the occurrence of a ferroelectric phase transition. The low temperature peak was suppressed significantly by oxygen-annealing and enhanced by argon-annealing, suggesting it is oxygen-vacancy related. The ceramics exhibited a single dielectric relaxation behavior, which was attributed to the bulk contribution according to the combination analysis of impedance and electric modulus frequency spectra. Kinetic analyses of ac dielectric data were carried out to probe the possible conduction–relaxation mechanisms. Above ∼400 °C, the relaxation and conduction of the compound was assigned to the motion of ionized oxygen vacancies. The experimental results indicated that the Gd-doping suppressed the leakage and improved dielectric properties of Na_0.5Bi_4.5Ti₄O₁₅ ceramics.