A computational study was carried out to investigate the effect of surface termination on Janus Hf2COS MXene by substituting partly the O-terminated layer with S atoms. Our predictions confirm that this chemical strategy allows one to tailor the band gap of MXenes. Indeed, the semiconducting character of Hf2CO2 MXene decreases by the exchange of O by S atoms. From a structural point of view, dynamical, mechanical, and thermal analysis confirm the thermodynamic stability of the Janus Hf2COS MXene, which shows metallic character. Furthermore, topological chemical analysis indicates an ionic nature of Hf2CO2 MXene that tends to be reduced by increasing the concentration of S atoms, promoting a covalent character. Shortly, the present study illustrates how the properties of MXenes can be tailored by functionalizing them with different chemical terminations.