© 2021 Elsevier B.V.In this study, plasmonic properties of transition metal atoms (Sc-Zn, Pd, Pt) adsorbed on various sites of graphene sheets were investigated before and after CO adsorption by density functional theory (DFT). To this aim, Electron Energy Loss Spectroscopy (EELS) of these systems were obtained by periodic DFT. The results indicated that the highest in-plane plasmon peaks undergo blue shifts after CO adsorption, in such a way that Co–graphene system demonstrates the highest blue shift of 2.42 eV from Visible to UV region. Also, out-of-plane plasmons do not exhibit significant sensitivity to CO adsorption. For deeper understanding of electronic structure of metal-graphene systems, appropriate molecular models were considered, and natural bond orbital (NBO) analysis applied. Comparison of solid-state net charges with the NBO charges showed that CO adsorption has more non-local characteristics, while the metal atom adsorption on bare graphene surface has more local nature. Moreover, Jahn–Teller distortion of metal adatom strongly depends on the net charges of nearest-neighbor C atoms on the surface. In addition, spin distribution on metal 3d orbitals plays a role to some extent. These results can be useful for experimental researchers who want to construct the sensors and optical devices based on metal-graphene interactions.