We present a simple and novel strategy to synthesize TiO2 nanoparticles (NPs) based on electrochemical anodization of a Ti wire in an aqueous KCl electrolyte. The Ti wire is very rapidly and directly converted to TiO2 NPs by the anodization process, allowing mass production of TiO2 NPs. The size of the synthesized NPs can be readily tuned by changing the concentration of the electrolyte. We found that the field-assisted etching related to a strong electric field and the rapid etching rate caused by chloride ions play important roles for the formation of TiO2 NPs. This approach can also be applied to the mass production of other semiconducting metal oxide NPs such as tungsten-oxide NPs. TiO2 NPs showed higher photocatalytic activity compared to Degussa (P 25) under the same conditions. The higher photocatalytic activity of TiO2 NPs is attributed to the polymorphism. We believe that our approach can be used in broad areas including biomedical applications, photovoltaics, optics, and electronics.