High-resolution electron energy loss spectroscopy has been used to study the electronic response of periodically rippled monolayer graphene grown on Ru(0001). A plasmonic mode, assigned to the π plasmon, has been observed at around 6 eV. The dispersion curve of this collective mode indicates plasmon confinement within the hills of the ripples. Moreover, we found that the corrugation of the graphene sheet also significantly affects the damping processes of the π plasmon.