NaNbO₃-based (NN) energy storage ceramics exhibit high breakdown electric field strength (E_b) with large recoverable energy storage density (W_rec). However, due to their large energy loss density (W_loss) under strong electric fields, maintaining high energy storage efficiency (η) is a challenge. In this study, to produce a dielectric ceramic with both high W_rec and η, a ternary system was designed. By the addition of (Bi_0.5Na_0.5)_0.7Sr_0.3TiO₃ (BNST), the grain size of 0.90NaNbO₃–0.10Bi(Mg_0.5Ta_0.5)O₃ (0.10BMT) was effectively reduced, and the long-range ordered structure was broken, providing an easily turned over dielectric domain to inhibit W_loss. The activation energy of the grain boundary increased with the increase in resistivity, indicating that the concentration of free vacancies at the grain boundary was low. The jump barrier of oxygen vacancies in the grain boundary increased, making up for the grain boundary defects, thus increasing E_b. When the BNST concentration increased, the E_b and W_rec of the dielectric ceramics increased. Optimum performance was obtained with the 0.75[0.90NaNbO₃–0.10Bi(Mg_0.5Ta_0.5)O₃]–0.25(Bi_0.5Na_0.5)_0.7Sr_0.3TiO₃ (0.25BNST) ceramic, which exhibited an exceptionally high E_b (800 kV cm⁻¹) and W_rec (8 J cm⁻³), while maintaining a relatively high η (90.4%). The ceramics developed in this study showed excellent temperature and frequency stability over 20–200 °C and 1–160 Hz, respectively. In addition, the dielectric properties of the ceramics were maintained after 10 000 hysteresis cycles. The 0.25BNST ceramic showed an exceptionally fast t_0.9 (∼32 ns) and a high C_D (614.5A cm⁻²). This study demonstrates that the energy storage performance and stability of the fabricated 0.25BNST ceramic are superior to those of previously reported dielectric ceramics.