A bis-hydroxo-bridged dinuclear Co-III-pyridylmethylamine complex (1) was synthesized and the crystal structure was determined by X-ray crystallography. Complex 1 acts as a homogeneous catalyst for visible-light-driven water oxidation by persulfate (S2O82-) as an oxidant with [Ru-II(bpy)(3)](2+) (bpy = 2,2'-bipyridine) as a photosensitizer affording a high quantum yield (44%) with a large turnover number (TON = 742) for O-2 formation without forming catalytically active Co-oxide (CoOx) nanoparticles. In the water-oxidation process, complex 1 undergoes proton-coupled electron-transfer (PCET) oxidation as a rate-determining step to form a putative dinuclear bis-mu-oxyl Co-III complex (2), which has been suggested by DFT calculations. Catalytic water oxidation by 1 using [Ru-III(bpy)(3)](3+) as an oxidant in a (H2O)-O-16 and (H2O)-O-18 mixture was examined to reveal an intramolecular O-O bond formation in the two-electron-oxidized bis-mu-oxyl intermediate, prior to the O-2 evolution.