Photo-thermal-pH triple-responsive hydrogels are special kinds of smart materials which can respond to changes in ambient temperature, pH, and light intensity simultaneously. In this paper, a field theory is developed for the photo-thermal-pH triple-responsive hydrogels by incorporating effects of dissociation of acidic groups and photo-thermal conversion into the thermodynamic theory of temperature-sensitive hydrogels. Constitutive equations are derived by means of the free energy imbalance inequality. The feasibility of the presented model is tested and validated by comparing the simulation results for stress-free swelling with the available experimental data. Moreover, the sensitivities of the presented model to changes in temperature, pH, and light intensity are further studied through the biaxial constrained swelling case, and the factors affecting deformation is investigated numerically.