TiO2 is a famous wide-bandgap semiconductor which has important applications in photocatalysis and photovoltaics. In the solar energy conversion process, photoexcited electrons can be trapped by the lattice distortion and can form a small polaron, which can quench through electron–hole recombination. On the TiO2 surface, the molecule adsorption is proposed to play an important role to influence the dynamics of a photoexcited polaron. In this study, by ab initio nonadiabatic molecular dynamics simulation, we study the dynamics of a photoexcited small polaron in the rutile TiO2(110) surface system and the effects of H2O adsorption. It is found that the photoexcited electron can be trapped by the lattice distortion in the subsurface layer within 25 fs. The H2O adsorption stabilizes the polaron by further stretching the Ti–O bonds around it. In a comparison with the photoexcited free electrons, the formation of the polaron shortens the carrier lifetime distinctly due to the reduction of the energy difference with the photoexcited hole at the valence band maximum, and reinforcement of electron–phonon (e–ph) coupling. Moreover, the H2O adsorption is found to further reduce the photoexcited polaron lifetime by enhancing the e–ph coupling with high-frequency phonons. This work suggests that molecule adsorption provides a new stratagem to tune the lifetime of a photoexcited small polaron.