We report a facile and effective low-temperature modification method for improving the photocatalytic activity of commercial Degussa P25 TiO2. The method involves the microwave treatment of TiO2 in an H2O2 solution. The properties of the modified TiO2, including improved crystallinity, size distribution, and aggregation of the anatase/rutile phases, were analyzed by X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscope (TEM). Further, we found that Ti3+ and Ti-OH were formed because of the presence of H2O2. Moreover, the valence band (VB) edge was determined by Raman and X-ray photoelectron spectroscopy (XPS) studies. In addition, photoluminescence (PL) measurements showed that the surface defects in the modified P25 affected the hole-electron recombination rate. UV-vis spectroscopy measurements show various size distribution and band gap effects after the simple modification process, and the photocatalytic activity was determined using the degradation of Rhodamine 6G (R6G). From these results, the degradation efficiency under solar and UV irradiation were measured to be 94 and 51% for modified P25 and be 47 and 25% for unmodified P25, respectively. The modified P25 showed twice the photocatalytic performance of the unmodified P25. The enhancement in the photocatalytic efficiency of modified P25 is correlated to the formation of surface defects, such as Ti3+ and Ti-OH, the size distribution, and the re-aggregation of the anatase/rutile phases.