Post-synthesis heat treatment provides a route for tuning the structural and optical response of TiO₂ without necessarily altering its particle architecture. In this study, TiO₂ nanostructures prepared by the sol–gel method were evaluated before and after a stepwise heat treatment from 250 to 400 °C using XRD/Rietveld, Raman spectroscopy, FE-SEM/EDS, statistical morphometry, and UV–Vis diffuse reflectance spectroscopy, with XPS performed on TiO₂-400. TiO₂-250 consisted of 96.76 ± 1.22 wt.% anatase and 3.24 ± 0.47 wt.% brookite, whereas anatase was the only crystalline phase detected in TiO₂-400 within the XRD detection limit. Concurrently, the coherent crystalline domain size increased from 44.3 ± 2.6 to 53.6 ± 1.9 nm, without a statistically significant increase in the equivalent circular diameter of the particles. The apparent optical band gap increased from 3.14 ± 0.03 to 3.25 ± 0.03 eV, while the Urbach energy decreased by approximately 48%, from 189.4 ± 1.1 to 98.0 ± 0.4 meV. These results show that moderate heat treatment is accompanied by polymorphic homogenization and predominantly intraparticle crystalline/electronic reorganization, together with narrowing of the band-tail distribution without appreciable external particle growth. Overall, the study establishes a multiscale correlation between phase evolution, coherent-domain growth, morphological stability, and optical-edge narrowing in sol–gel TiO₂ nanostructures.