Nanowire/nanorod TiO2 structures of approximately 8 nm in diameter and around 1,000 nm long were synthesized by alkaline hydrothermal treatment of two different TiO2 nanopowders. The first precursor was TiO2 obtained by the sol-gel process (SG-TiO2); the second was the well-known commercial TiO2 P-25 (P25-TiO2). Anatase-like 1D TiO2 nanostructures were obtained in both cases. The one-dimensional (1D) nanostructures synthesized from SG-TiO2 powders turned into rod-like nanostructures after annealing at 400 °C for 2 h. Conversely, the nanostructures synthesized from P25-TiO2 preserved the tubular structure after annealing, displaying a higher Brunauer–Emmett–Teller surface area than the first system (279 and 97 m2/g, respectively). Despite the higher surface area shown by the 1D nanostructures, in both cases the photocatalytic activity was lower than for the P25-TiO2 powder. However, the rod-like nanostructures obtained from SG-TiO2 displayed slightly higher efficiency than the sol-gel prepared powders. The lower photocatalytic activity of the nanostructures with respect to P-25 can be associated with the lower crystallinity of 1D TiO2 in both materials.