We report on the observation of a low-energy surface plasmon mode in the case of a highly ordered organic monolayer adsorbed onto a metallic substrate. Careful investigation using high-resolution electron energy loss spectroscopy, tuning the kinematics, demonstrates that this mode presents a strong energy dependence with the transferred momentum. We treated this mode using a Drude ansatz and discussed its positive quadratic dispersion. We established the physical origin of this surface plasmon as due to a space-charge region arising from an electron-vibration coupling at the interface between the organic monolayer and the metallic substrate. The existence of a space-charge region at an organic-metal interface is of particular interest as it brings a new enlightenment in the understanding of the physical mechanisms involved at such interfaces, of which charge injection plays a decisive role for device performances.