We, herein, present comparative investigations on the Na0.5Bi0.5Cu3Ti4O12 ceramic samples with and without 10 mol% excess of Na/Bi. The samples were prepared by the standard solid-state reaction technique. The dielectric properties of the sample were investigated in the temperature (93–320 K) and frequency (20 Hz–10 MHz) windows. Three thermally activated dielectric relaxations observed in Na0.5Bi0.5Cu3Ti4O12 with the activation energies of 0.104, 0.267, and 0.365 eV for the low-, middle-, and high-temperature dielectric relaxations, respectively. Only the low-temperature relaxation was observed in both Na and Bi excessive samples. X-ray photoemission spectroscopy results revealed the mixed-valent structures of Cu+/Cu2+ and Ti3+/Ti4+ in Na0.5Bi0.5Cu3Ti4O12 sample, but only Ti3+/Ti4+ in Na and Bi excessive samples. Our results showed that the dielectric properties of the investigated samples are strongly linked with these mixed-valent structures. The high- and low-temperature relaxations were attributed to be a polaron-type relaxation due to localized carriers hopping between Cu+/Cu2+ and Ti3+/Ti4+, respectively. The middle-temperature relaxation is suggested to be a dipole-type relaxation caused by the defect complex of bismuth and oxygen vacancies.