The intrinsic capability of the paddlewheel-type dirhodium tetraacetate complex, [Rh2(O2CCH3)4(H2O)2] ([1(H2O)2]), as a hydrogen evolution catalyst (HEC) for photochemical hydrogen evolution from aqueous solution was illustrated. This was achieved by using an optimized artificial photosynthesis (AP) system with a cyclometalated iridium complex [Ir(ppy)2(bpy)](PF6) ([Ir-PS-1]) and triethylamine (TEA) serving as a photosensitizer (PS) and a sacrificial donor, respectively. The total amount of hydrogen evolution and the turnover number (TON) of catalysis using this AP system were 385.7 μmol and 3857 (per Rh ion), respectively; these values are higher than those of [Rh(dtBubpy)3](PF6)3, which is the most efficient HEC among the mononuclear rhodium complexes, and RhCl3. Moreover, the catalytic performance of [1(H2O)2] was further accelerated by using [Ir(ppy)2(dtBubpy)](PF6) [Ir-PS-3] as a PS; 9886 TON (H2 per Rh ion) was verified after 12 h of irradiation. In addition, the detailed mechanism of hydrogen evolution catalyzed by [1(H2O)2] was clarified by combining electro- and photochemical analyses and DFT calculations. The optimized geometries of [1(H2O)2], [1], hydride intermediates [H-Rh2(O2CCH3)4] ([H-1]), and their reduced species were theoretically verified by DFT calculations. Moreover, their redox potentials were theoretically estimated and compared with the observed potentials. Their combination analyses indicated that (i) the formation of [1], which has an open-metal site for hydrogen evolution and can be reduced by the one-electron reduced species of [Ir-PS-1], is a trigger for hydrogen evolution; (ii) [H-1] and its reduced species, which are verified by CV analyses, are key intermediate species in this reaction; and (iii) photochemical hydrogen evolution catalyzed by [1(H2O)2] occurred by two-electron reduction processes.