After the Fukushima nuclear accident, tremendous efforts were made to treat radiocesium, radiostrontium, and other radioactive materials. For the first time, we demonstrate that a TiO2 photocatalyst can significantly enhance Cs adsorption by Prussian blue-deposited TiO2 (PB/TiO2) under UV irradiation. In this study, we synthesized PB/TiO2 using the photodeposition method. After the Cs ions were adsorbed on the PB/TiO2 in darkness, we then exposed the PB/TiO2 to UV light irradiation. This resulted in a further increase in Cs ion adsorption of more than 10 times the amount adsorbed in darkness. This photocatalytic-enhanced adsorption of Cs ions was not observed on PB mixed with SiO2, nor under visible light irradiation. We investigated the effects of PB concentration, PB/TiO2 concentration, and gas purging on both dark and photocatalytic-enhanced adsorption of Cs ions by PB/TiO2. Based on the results, we suggest that the photocatalytic-enhanced adsorption of Cs ions on PB/TiO2 is due to photocatalytic reduction of PB, which leads to additional adsorption of Cs ions. The change in solution color before and after the reaction, and the change in solution pH in the dark and during UV irradiation strongly support this suggestion. The photocatalytic-enhanced adsorption of Cs ions was equivalent during radioactive Cs-137 removal, indicating important applications for pollutant removal from contaminated water.