In this study, we investigate the piezoelectric characteristics of 0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr,Ti)O₃ (PNN-PZT) with MnO₂ additive (0, 0.25, 0.5, 1, 2, and 3 mol%). We focus on the fabrication of a piezoelectric ceramic for use as both actuator and sensor for ultrasound transducers. The actuator and sensor properties of a piezoelectric ceramic depend on the piezoelectric strain coefficient d and piezoelectric voltage coefficient g, related as g = d/ε^T. To increase g, the dielectric constant ε^T must be decreased. PNN-PZT with MnO₂ doping is synthesized using the conventional solid-state reaction method. The electrical properties are determined based on the resonant frequencies and vibration modes measured by using an impedance analyzer. The MnO₂ addition initially improves the tetragonality of the PNN-PZT ceramic, which then saturates at a MnO₂ content of 1 mol%. Therefore, the dielectric constant and piezoelectric coefficient d₃₃ steadily decrease, while the mechanical properties (Q_m, Young’s modulus), tanδ, electromechanical coupling coefficient k, and piezoelectric voltage coefficient g were improved at 0.5–1 mol% MnO₂ content.