The photocatalytic reduction of Ag+ ions by TiO2 nanoparticles was examined in water under high-temperature high-pressure conditions of T = 25-400 degrees C and P = 30 MPa. The analysis of the reaction by uv-vis absorption spectroscopy, the atomic absorption spectrometry, and TEM observations revealed that the efficient photocatalytic activity compared with that at room temperature remained even under hydrothermal and supercritical conditions. These results could open the way to new applications of TiO2 under extreme environments which have never been explored before.