We investigated the polaron-assisted nonadiabatic dynamics in protonated TiO2, as well as the polaron-H2O coupling and its effects on the relaxation of photogenerated electrons. We observed that different polaron hopping regimes result in varied nonadiabatic couplings and relaxations of excited electrons from the conduction band minimum to the gap states of protonated TiO2, with a weak dependence on the actual trapping site of the polaron. Surface-adsorbed H2O molecules can attract polarons toward the adsorbed Ti sites, with the coupling between H2O and the polaron being inversely proportional to their distance. Our findings suggest that the lifetime of the photogenerated charge carriers can be extended by reducing the polaron-H2O distances, with expected benefits to the efficiency of the reduced TiO2 samples for photocatalytic applications.