TiO₂ nanotubes structures were synthesized by alkaline hydrothermal treatment of commercial TiO₂ powder, followed by HCl treatment and thermal annealing. The effects of acid-treatment time and annealing temperature on the structural, morphological, and optical properties were investigated by XRD, SEM, size-distribution analysis, and UV–Vis diffusereflectance spectroscopy. XRD showed that 1 h HCl treatment was insufficient to obtain the characteristic anatase TiO₂ structurephase, whereas extending the treatment to 3 h resulted in clear anatase phase formation. Subsequent annealing improved crystallinity, with the crystallite size increasing from 13.44 to 15.12 nm between 450 and 650 °C. SEM confirmed the transformation of the commercial precursor particles into nanotubes structures with an average diameter of 18.4 nm. The optical band gap of the 3 h HCl-treated samples decreased from 3.45 eV before annealing to approximately 3.32 eV after thermal treatment. These results demonstrate that acid-treatment duration is a critical parameter for phase formation, while annealing effectively controls the crystallinity and optical properties of hydrothermally synthesized TiO₂ nanotubes.