High-resolution electron energy loss spectroscopy has been used to study the influence of a humid environment on the collective electronic excitations of a quasifreestanding graphene monolayer supported on Pt (111). We found that the adsorption of water molecules at room temperature on graphene/Pt (111) is dissociative and it gives rise to adsorbate fragments. Our results indicate that these adsorbed species change the energy, the dispersion, and the lifetime of the graphene sheet plasmon. Moreover, the phase velocity of this low-energy collective mode decreases by 11% even if its lifetime notably increases. By contrast, humidity has a negligible effect on the energy of the π plasmon at 6 eV and the unique effect induced by humidity is a significant attenuation of its intensity. Such information is essential for tailoring graphene-based plasmonic devices which should operate in ambient air humidity.