In recent years, TiO₂-based giant dielectric ceramics have shown broad application prospects in the field of electronic devices owing to their excellent giant dielectric properties. Nevertheless, its practical application is hindered by substantial dielectric loss and inadequate temperature/frequency stability. This study introduced co-doping with Sb⁵⁺ and Gd³⁺ to prepare (Sb_0.5Gd_0.5)_xTi_1–xO₂ (SGTOx, x = 0, 0.01, 0.02, 0.03, 0.05) co-doped TiO₂ ceramics. By constructing electron-pinned defect clusters, synergistic optimization of a high dielectric constant and ultra-low dielectric loss was achieved. The SGTO0.01 ceramics exhibited outstanding comprehensive performance, possessing a high dielectric constant of 1.06 × 10⁴ and an ultra-low dielectric loss of 0.011 at 1 kHz, along with excellent frequency stability (20 Hz – 1 MHz) and temperature stability (−150 ℃ to 260 ℃, Δε_r < ±15%), as well as superior DC bias characteristics (0–26 V). X-ray photoelectron spectroscopy (XPS) and dielectric response analysis indicate that the synergistic doping of Sb⁵⁺ and Gd³⁺ promotes the formation of strongly coupled defect clusters. The defect dipole response constitutes the primary source of the high dielectric constant, while redundant internal barrier layer capacitance (IBLC) effects and electrode interface polarization are observed at medium-to-low frequencies. The electronic pinning effect of defect dipoles enhances electron localization and effectively suppresses long-range carrier transport. This study provides new insights into defect engineering for developing high-performance giant dielectric materials and clarifies the structure–property relationship between defect cluster configurations and dielectric performance in heterovalent co-doping systems.