There is ongoing debate as to when oxygenic photosynthesis evolved on Earth. Geochemical data from ancient sediments indicate localised or ephemeral photosynthetic O2 production prior to Great Oxidation Event (GOE) ca. 2.5-2.3 billion years (Ga) ago, and currently suggest Archean origins, approximately 3 Ga ago or earlier. However, sedimentary records of the early Earth often suffer from preservation issues, and poor control on the timing of oxidation leaves geochemical proxy data for the ancient presence of O2 open to critique. Here we report rare earth element data from three different Archean carbonate platforms preserved in greenstone belts of the NW Superior Craton (Canada) that were deposited by the activity of marine photosynthetic bacteria at 2.87 Ga, 2.85 Ga, and 2.78 Ga. All three indicate O2 production before the GOE in the form of significant depletions in cerium (Ce), reflecting oxidative Ce removal from ancient seawater, as occurs today. Using 138 La-138 Ce geochronology, we show that La/Ce fractionation, and thus Ce oxidation, occurred at the time of deposition, making these the oldest directly dated Ce anomalies. These results place the origin of oxygenic photosynthesis in the Mesoarchean or earlier and bring important new perspective on a long-standing debate regarding Earth's biological and geochemical evolution.