Gd, N, and P tri-doped anatase-TiO2 (GNPTO) nano-photocatalyst has been synthesized from TiCl4 hydrolysis using gadolinium nitrate, urea, and phosphoric acid as dopant sources via a hydrothermal process followed by calcination at 400 degrees C. The physicochemical properties of as-synthesized samples have been investigated by X-ray diffraction, transmission electron microscopy, N-2 physical adsorption-desorption, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, UV-vis absorbance spectroscopy, and photoluminescence spectroscopy techniques. Doped P in the TiO2 lattices plays a predominant role in inhibiting the phase transformation and crystal growth, and in improving the surface textural properties. Gd, N, and P tri-doping inhibits the recombination of photogenerated electrons and holes, increases the surface hydroxyls, and improves the UV absorption. GNPTO exhibits the highest photoactivity towards 4-chlorophenol (4-CP) degradation under simulated sunlight irradiation among the undoped, doped, and P25 TiO2. center dot OH radicals play a key role in 4-CP degradation, and photogenerated electrons also play a minor role through the final formation of center dot OH via H2O2. Synergetic mechanism of the enhanced photoactivity for GNPTO has also been discussed.