Regulation of surface acidity is critical for steering reaction pathways in catalytic biomass conversion; however, the modulation of Lewis acidity in MXene-supported single-atom catalysts remains poorly understood. In this work, density functional theory calculations were performed to systematically investigate how surface terminations coupled with metal identity govern the Lewis acidity of single-atom sites on Ti₃C₂-based MXenes. Two representative terminations (-O and -OH) were considered, and various metal atoms were anchored to construct single-atom catalysts. Formation energies were evaluated to assess thermodynamic stability, while NH₃ adsorption energies were employed as a descriptor for Lewis acidity. The results reveal a pronounced termination-dependent modulation of acidity. Specifically, -OH termination disfavors single-atom stabilization, whereas -O termination ensures strong anchoring. Electronic structure analysis indicates that enhanced acidity originates from termination-induced electronic polarization and charge redistribution. This work establishes a structure–termination–acidity relationship and provides theoretical guidance for the rational design of MXene-based catalysts with tunable acidity.