Aiming at boosting the photocatalytic activities of rutile and anatase TiO2 surfaces, an in-depth investigation of stoichiometric and reduced rutile TiO2 (1 1 0) and anatase TiO2 (1 0 1) decorated with bipyramidal and trapezoidal Ag5 atomic quantum clusters (AQCs) is carried out. In this study, density functional theory (DFT) plus a Hubbard correction (U) is implemented to explore the geometric and electronic properties of such systems. It is found that the silver AQCs donate electrons to both stoichiometric and reduced TiO2 surfaces resulting in the formation of a single polaron at either a fivefold coordinated (Ti5c) atom or a sixfold coordinated (Ti6c) atom, indicating improved surface activity. Depositing Ag5 AQCs on both TiO2 surfaces can produce mid-gap states within the band gap of the bulk, thereby improving the optical response of the composite in the visible and infrared. As expected, the number of mid-gap energy states increases further by introducing a single oxygen vacancy into the studied surfaces, which means that Ag5 AQCs and oxygen vacancies can reinforce each other, leading to higher efficient photocatalytic activity. We also find that upon adsorption of Ag5 AQCs on an anatase TiO2 (1 0 1) surface, the energy required to form an oxygen vacancy is lower than that of rutile TiO2 (1 1 0). Moreover, the adsorption of both bipyramidal and trapezoidal Ag5 AQCs on both TiO2 surfaces generally leads to significant distortion of the clusters, which accounts for the significant reduction in the total energy as compared to the pristine TiO2. This detailed investigation provides insight into new mechanisms for enhancing photocatalytic efficiency of both rutile TiO2 (1 1 0) and anatase TiO2 (1 0 1) surfaces.