Recently, a MoSi₂N₄ monolayer has been successfully synthesized by a delicately designed chemical vapor deposition (CVD) method. It exhibits promising (opto)electronic properties due to a relatively narrow bandgap (∼1.94 eV), high electron/hole mobility, and excellent thermal/chemical stability. Currently, much effort is being devoted to further improving its properties through engineering defects or constructing nanocomposites (e.g., van der Waals heterostructures). Herein, we report a theoretical investigation on hydrogenation as an alternative surface functionalization approach to effectively manipulate its electronic structures and optical properties. The calculation results suggested that chemisorption of H atoms on the top of N atoms on MoSi₂N₄ was energetically most favored. Upon H chemisorption, the band gap values gradually decreased from 1.89 eV (for intrinsic MoSi₂N₄) to 0 eV (for MoSi₂N₄-16H) and 0.25 eV (for MoSi₂N₄-32H), respectively. The results of optical properties studies revealed that a noticeable enhancement in light absorption intensity could be realized in the visible light range after the surface hydrogenation process. Specifically, full-hydrogenated MoSi₂N₄ (MoSi₂N₄-32H) manifested a higher absorption coefficient than that of semi-hydrogenated MoSi₂N₄ (MoSi₂N₄-16H) in the visible light range. This work can provide theoretical guidance for rational engineering of optical and optoelectronic properties of MoSi₂N₄ monolayer materials via surface hydrogenation towards emerging applications in electronics, optoelectronics, photocatalysis, etc.