Na0.5Y0.5Cu3Ti4-xNbxO12 powders with x values of 0.0, 0.025, and 0.03 were successfully synthesized using the solid-state reaction method. The sintered specimens consistently exhibited a pure phase of Na0.5Y0.5Cu3Ti4O12 (NYCTO) and experienced a slight unit cell expansion due to the ionic size difference between Ti4+ and Nb5+ ions. The addition of Nb5+ resulted in a decrease in the average grain size of NYCTO, which can be attributed to the introduction of defects and the solute drag effect. An even elemental distribution was maintained across the samples, with only minor copper segregation observed at the grain boundaries. Importantly, Nb5+ doping markedly improved the dielectric properties of NYCTO, achieving a large dielectric constant of 104, while also reducing the loss tangent (∼0.02). Impedance, modulus, and admittance spectroscopy were employed to analyze the electrical characteristics of both the grains and the grain boundaries. The electron transfer dynamics of Cu+/Cu2+ and Ti3+/Ti4+ were investigated to elucidate the grain conduction mechanisms. The incorporation of Nb5+ significantly improved the electrical properties at the grain boundaries, which in turn enhanced the overall dielectric performance of the material.