Gold nanomaterials exhibiting size-dependent optical and catalytic properties are extensively used in sensors, catalysts, and photodevices. Yet, the size effect of gold nanoclusters (AuNCs) on O production has rarely been explored. This study demonstrates that lysozyme (LYZ)-stabilized Au clusters could serve as a photosensitizer for the efficient production of O under the irradiation of green light-emitting diode relative to the LYZ-stabilized Au clusters and gold nanoparticles (AuNPs; approximately 3.2 nm). The same stabilizing macromolecule, LYZ, was used as a template for preparing AuNCs, enabling the comparison of their O production efficiency resulting from ligand effects without complications. 9,10-Anthracenediyl-bis(methylene)dimalonic acid was used to detect O by monitoring its absorption peak intensity with time. The QYs for O generation were determined to be 16.2%, 3.0%, and <0.1% for the LYZ-stabilized Au clusters, Au clusters, and AuNPs, respectively, using Rose Bengal (RB) as a reference. Electron spin resonance spectroscopy confirmed that the photoexcitation of Au clusters afforded O rather than other reactive oxygen species, and their O production efficiency increased with decreasing excitation wavelength. The mechanism behind these observations indicated that the positive surface charge of the LYZ-stabilized Au clusters at pH 5.0 promoted the approach of O molecules to the cluster surface and thus facilitated efficient O generation. Additionally, the relatively large surface-to-volume ratio of the LYZ-stabilized Au clusters, compared to that of the LYZ-stabilized Au clusters, could be a contributing factor for enhanced O generation.