Seawater is a valuable water resource in coastal regions. However, during seawater chlorination, a group of halophenols (HPs) may be formed. These HPs have lower odor and taste detection thresholds than other disinfection by-products (DBPs), however these are usually more toxic than most of the abundantly detected DBPs. Hence, an effective approach for control of HP formation during seawater chlorination is required to minimize highly toxic HP formation. Pretreatment using TiO2 photocatalysis was applied in this study to assess its ability for removal of HP precursors. Seawater samples with external addition of 1 mg/L phenol were spiked with TiO2 from 0.1 to 10.0 g/L and exposed under UV light for 2 to 120 min. The UV absorbance at 254 nm and the excitation–emission matrix fluorescence of dissolved organic matter were measured for each treated sample. It was observed that the optimal treatment condition to achieve the highest UV254 removal was 4.0 g/L TiO2 with UV exposure of 30 min. By pretreatment using this method and stated dose and exposure, only two types of HPs were detected during chlorination, compared with four types of HPs formed in the untreated samples. Moreover, the pretreatment greatly reduced the concentration of 2,4,6-TBP from more than 400 μg/L to less than 1 μg/L. The significance of this research study is to identify the effectiveness of UV/TiO2 in reducing DBP formation by analyzing the mechanisms during the process, which indicates the use of UV/TiO2 pretreatment for control of HP formation in seawater during chlorination.