Developing high-performance photoanodes is a critical issue in photoelectrochemical (PEC) water splitting. Herein, we report an effective approach to improve the PEC performance of TiO2 nanowire (NW) photoanode by introducing oxygen defects with a facile acid treatment method. The introduced oxygen defects serve as electron donors and lead to an enhancement in the donor density, which facilitates the charge transport in TiO2 NWs. The acid-treated TiO2 (A-TiO2) photoanode achieves a photocurrent density of 2.8 mA/cm(2) at 0.23 V vs. Ag/AgCl, which is 2.5 times higher than that of the pristine TiO2 photoanode. This work demonstrated that introducing oxygen defects through a facile acid treatment method can effectively improve the PEC performance of conventional TiO2 photoanodes. It provides a promising solution to fabricate high-performance PEC photoanodes. (C) 2018 The Electrochemical Society.