Photocatalytic oxygenation of hexamethylbenzene occurs under visible-light irradiation of an O(2)(-)saturated acetonitrile solution containing a cobalt porphyrin complex Co-II(TPP) (TPP2 = tetraphenylporphyrin dianion), water, and triflic acid (HOTf) via a one-photon-two-electron process, affording pentamethylbenzyl alcohol and hydrogen peroxide as products with a turnover number of >6000; in this reaction, H2O and O-2 were used as an oxygen source and a two-electron oxidant, respectively. The photocatalytic mechanism was clarified by means of electron paramagnetic resonance, time-resolved fluorescence, and transient absorption measurements as well as O-18-labeling experiments with (H2O)-O-18 and O-18(2). To the best of our knowledge, we report the first example of efficient photocatalytic oxygenation of an organic substrate by a metal complex using H2O as an oxygen source and O-2 as a two-electron oxidant.