High-quality BaTiO₃ nanopowders were synthesized via a sol–gel method using butyl titanate and barium serving as precursors. This study systematically investigates the influence of calcination temperature (600–1000 °C) and gel aging time (2–10 h) on the phase evolution and microstructure of the nanoparticles. A pure tetragonal phase with a high tetragonality (c/a ratio of 1.0100) and an average particle size of 140 nm was achieved at 1000 °C. X-ray photoelectron spectroscopy and Ultraviolet–Visible diffuse reflectance spectroscopy analyses revealed that high-temperature calcination induced the formation of oxygen vacancies and Ti³⁺ defects, leading to a narrowing of the optical bandgap from 3.01 eV to 2.98 eV. An optimal aging time of 4 h yielded uniform nanoparticles with a high specific surface area, whereas prolonged aging (>6 h) resulted in the re-emergence of BaCO₃ impurities and severe agglomeration due to the formation of a rigid gel network. This work provides a precise processing window for fabricating high-purity, highly tetragonal BaTiO₃ nanopowders suitable for the next generation of miniaturized electronic devices.